Rules coming from episodes are considered "canon", and are given higher precedence than Rules coming from comics, novels, and other sources.

1.

 Once you have their money, you never give it back.
 Rom, "The Nagus"

1.

 If they want their money back, give it to them.
 Rom and Quark, "Prophet Motive"

3.

 Never spend more for an Acquisition than you have to.
 Quark, "The Maquis, Part II"

4.

 A woman wearing clothes is like a man in the kitchen.
 'Rules of Netquisition' #1

6.

 Never allow family to stand in the way of opportunity.
 Zek, "The Nagus"

7.

 Keep your ears open.
 Odo, "In the Hands of the Prophets"

8.

 Only a fool passes up a business opportunity.
 Vung, Warchild

8.

 Small print leads to large risk.
 Rules

9.

 Opportunity plus instinct equals profit.
 Nog, "The Storyteller"

10.

 Greed is eternal.
 Quark, "Prophet Motive"

10.

 Greed is dead.
 Rom, "Prophet Motive"

12.

 Anything worth doing is worth doing for money.
 'Rules of Netquisition' #1

13.

 Anything worth doing is worth doing twice.
 The Making of ST:DS9

16.

 A deal is a deal (...until a better one comes along).
 Quark, "Melora" (Add-on: Rules)

17.

 A contract is a contract is a contract--but only between Ferengi.
 Rom, Quark, Brunt, and Gint, "Body Parts"

18.

 A Ferengi without profit is no Ferengi at all.
 Ben Sisko and Nog, "Heart of Stone"

19.

 Satisfaction is not guaranteed.
 Quark, "Meridian"

20.

 Only give money to people you know you can steal from.
 'Rules of Netquisition' #2

21.

 Never place friendship before profit.
 Quark, "Rules of Acquisition"

21.

 Never place profit before friendship.
 Rom, "Prophet Motive"

22.

 A wise man can hear profit in the wind.
 Pel, "Rules of Acquisition"

22.

 Latinum tarnishes, but family is forever.
 Rom, "Prophet Motive"

23.

 Money can never replace dignity.
 Rom, "Prophet Motive"

24.

 Latinum can't buy happiness, but you can sure have a blast renting it.
 'Rules of Netquisition' #2

25.

 There's always a way out.
 'Rules of Netquisition' #1

26.

 As the customers go, so goes the wise profiteer.
 'Rules of Netquisition' #1

27.

 There's nothing more dangerous than an honest businessman.
 Rules

31.

 Never make fun of a Ferengi's mother.
(...insult something he cares about, instead).
 Quark, "The Siege" (Add-on: Rules)

33.

 It never hurts to suck up to the boss.
 Quark and Pel, "Rules of Acquisition"

34.

 War is good for business.
 Quark, "Destiny"

35.

 Peace is good for business.
 Dax, "Destiny"

39.

 Friendship is temporary; profit is forever.
 'Rules of Netquisition' #1

40.

 She can touch your lobes, but never your Latinum.
 Rules

41.

 Profit is its own reward.
 Rules

42.

 What's mine is mine, and what's yours is mine too.
 'Rules of Netquisition' #1

44.

 Never confuse wisdom with luck.
 Rules

45.

 Ambition knows no family.
 'Rules of Netquisition' #2

46.

 Make your shop easy to find.
 'Rules of Netquisition' #1

47.

 Never trust anyone whose suit is nicer than your own.
 Quark, "Rivals"

48.

 The bigger the smile, the sharper the knife.
 Pel, "Rules of Acquisition"

49.

 Everything is worth something to somebody.
 'Rules of Netquisition' #2

50.

 Gratitude can bring on generosity.
 'Rules of Netquisition' #1

52.

 Never ask when you can take.
 Rules

57.

 Good customers are as rare as Latinum. Treasure them.
 Quark, "Armageddon Game"

58.

 There is no substitute for success.
 Rules

59.

 Free advice is seldom cheap.
 Quark, "Rules of Acquisition"

60.

 Let's you and him fight.
 Tunk, Balance of Power

60.

 Keep your lies consistent.
 Rules

62.

 The riskier the road, the greater the profit.
 Pel, "Rules of Acquisition"
Quark, "Little Green Men"

65.

 Win or lose, there's always Huyperian beetle snuff.
 Rules

68.

 Ear stroking will get you anything.
 'Rules of Netquisition' #1

69.

 Ferengi are not responsible for the stupidity of other races.
 Tunk, Balance of Power

72.

 Never trust your customers.
 'Rules of Netquisition' #1

73.

 If it gets you profit, sell your own mother.
 'Rules of Netquisition' #1

75.

 Home is where the heart is, but the stars are made of Latinum.
 Quark, "Civil Defense"

76.

 Every once in a while declare peace--
It confuses the hell out of your enemies.
 Quark, "The Homecoming"

77.

 It's better to swallow your pride than to lose your profit.
 'Rules of Netquisition' #2

79.

 Beware of the Vulcan greed for knowledge.
 Rules

82.

 The flimsier the product, the higher the price.
 Rules

84.

 A friend is not a friend if he asks for a discount.
 'Rules of Netquisition' #2

85.

 Never let the competition know what you're thinking.
 Rules

87.

 A friend in need means three times the profit.
 'Rules of Netquisition' #2

89.

 Ask not what your profits can do for you;
ask what you can do for your profits.
 Rules

94.

 Females and finances don't mix.
 Rules

96.

 For every Rule, there is an equal and opposite Rule.
(except when there's not)
 Rules

97.

 Enough... is never enough.
 Rules

98.

 Act without delay! The sharp knife cuts quickly.
 'Rules of Netquisition' #1

99.

 Trust is the biggest liability of all.
 Rules

100.

 If they take your first offer, you either asked too little or offered too much.
 'Rules of Netquisition' #2

101.

 The only value of a collectible is what you can get somebody else to pay for it.
 'Rules of Netquisition' #1

102.

 Nature decays, but Latinum is forever.
 Quark and Nog, "The Jem'Hadar"

103.

 Sleep can interfere with... [cut off]
 Pel, "Rules of Acquisition"

104.

 Faith moves mountains... of inventory.
 Rules

105.

 Don't trust anyone who trusts you.
 'Rules of Netquisition' #2

106.

 There is no honor in poverty.
 Rules

107.

 A warranty is valid only if they can find you.
 'Rules of Netquisition' #2

109.

 Dignity and an empty sack is worth the sack.
 Quark, "Rivals"

111.

 Treat people in your debt like family--exploit them.
 Ben Sisko, "Past Tense, Part I"

112.

 See Vidiot's site
 Quark, "Playing God"

113.

 See Vidiot's site
 Rules

116.

 There's always a catch.
 'Rules of Netquisition' #1

119.

 Never judge a customer by the size of his wallet
(...sometimes, good things come in small packages).
 'Rules of Netquisition' #1

121.

 Everything is for sale, including friendship.
 Rules

123.

 Even a blind man can recognize the glow of Latinum.
 Rules

125.

 Count it.
 'Rules of Netquisition' #1

127.

 Stay neutral in conflict so that you can sell supplies to both sides.
 'Rules of Netquisition' #2

135.

 Never trust a beneficiary.
 'Rules of Netquisition' #1

139.

 Wives serve. Brothers inherit.
 Odo, "Necessary Evil"

141.

 Only fools pay retail.
 Rules

144.

 There's nothing wrong with charity...
as long as it winds up in your pocket.
 Rules

146.

 Necessity, n. The mother of invention. Profit is the father.
 'Rules of Netquisition' #1

152.

 A lie is a way to tell the truth to someone who doesn't know.
 'Rules of Netquisition' #2

153.

 Sell the sizzle, not the steak.
 Quark, DS9 Comic #22

162.

 Even in the worst of times, someone turns a profit.
 Rules

169.

 Competition and fair play are mutually exclusive.
 'Rules of Netquisition' #1

177.

 Know your enemies... but do business with them always.
 Rules

181.

 Not even dishonesty can tarnish the shine of profit.
 Rules

184.

 [not given]
 Tunk, Balance of Power

188.

 A fool and his money is the best customer.
 'Rules of Netquisition' #2

189.

 Let others keep their reputation. You keep their money.
 Rules

191.

 A Ferengi waits to bid until his opponents have exhausted themselves.
 Tunk, Balance of Power

192.

 Never cheat a Klingon... unless you're sure you can get away with it.
 Rules

194.

 It's always good business to know about new customers before they walk in your door.
 Quark, "Whispers"

200.

 If you're going to have to endure, make yourself comfortable.
 'Rules of Netquisition' #1

202.

 The justification of profit is profit.
 Rules

203.

 New customers are like razor-backed Greeworms--
They can be succulent, but sometimes they bite back!
 Quark, "Little Green Men"

211.

 Employees are the rungs on the ladder of success--
don't hesitate to step on them.
 Grimp, "Bar Association"

214.

 Never begin a business negotiation on an empty stomach.
 Quark, "The Maquis, Part I"

216.

 Never gamble with an empath.
 Quark, The Laertian Gamble

217.

 You can't free a fish from water.
 Quark, "Past Tense, Part I"

218.

 Sometimes what you get free costs entirely too much.
 Quark, DS9 Comic #12: "Baby on Board"

219.

 Always know what you're buying.
 Rules [Quark referenced in "The Abandoned"]

219.

 Possession is 11/10 of the law. Tunk, Balance of Power

223.

 Beware the man who doesn't make time for oo-mox.
 Rules

229.

 Latinum lasts longer than lust.
 Rules

231.

 There's a sucker born every minute;
be sure you're the first to find each one.
 'Rules of Netquisition' #2

236.

 You can't buy fate.
 Rules

239.

 Never be afraid to mislabel a product.
 Gint, "Body Parts"

241.

 Never trust a hardworking employee.
 'Rules of Netquisition' #2

242.

 More is good... all is better.
 Rules

243.

 [not given]
 Quark, Devil in the Sky

255.

 A wife is a luxury... a smart accountant, a necessity.
 Rules

260.

 Life's not fair. How else would you turn a profit?
 'Rules of Netquisition' #2

261.

 A wealthy man can afford anything except a conscience.
 Rules

262.

 A verbal contract isn't worth the paper it's written on.
 'Rules of Netquisition' #2

263.

 Never allow doubt to tarnish your lust for Latinum.
 Rom, "Bar Association"

265.

 The customer is always right (...until you get their cash).
 'Rules of Netquisition' #1

266.

 When in doubt, lie.
 Rules

270.

 In business deals, a disruptor can be almost as important as a calculator.
 'Rules of Netquisition' #2

273.

 Never trust a hardworking employee.
 'Rules of Netquisition' #2

284.

 Deep down, everyone's a Ferengi.
 Rules

285.

 No good deed ever goes unpunished.
 Quark, "The Collaborator"

285.

 A good deed is its own reward.
 Rom, "Prophet Motive"

286.

 When Morn leaves, it's all over.
 Quark, "The House of Quark"

?.

 One person's secret is another person's opportunity.
 Quark, Devil in the Sky

Notes on the Rules

In the TNG episode "Bloodlines", Cmdr. Riker mentions that the Ferengi government is debating an amendment to the Rules of Acquisition.

We know from Quark ("The Maquis, Part I") and from Pel ("Rules of Acquisition") that there are 285 Rules of Acquisition.

Quark mentioned Rule #223 in "Profit and Loss".

Pel may have invented what she said for the Rule #103.

Quark may have violated Rule #112 when he was younger.

The second Rules numbered 1, 10, 21, 22, and 285, and the Rule listed as number 23, are the Grand Nagus' revised Rules in "Prophet Motive". These revised Rules were never officially released.

Pocket Books printed a book called The Ferengi Rules of Acquisition which included most of the Rules from the series and added many more. This source is listed here as simply Rules.

The most recent list of added Rules is from the Internet Competition, "The Rules of Netquisition". This contest was originally run in the Spring of 1994 on the rec.arts.startrek.current newsgroup. 21 Rules were taken from that competition to be included in this list. The contest was run again in the Spring of 1996, and 19 Rules were taken from that competition.

"Why did Quark travel back in time to 15th century Spain?"
"He wanted to learn the Rules of Inquisition." :-)