Mercantile Skill (3.5e Variant Rule)
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 Mercantile Skill
The Mercantile skill is applied whenever characters are buying or selling equipment. Mercantile is a Charisma-based skill that can be used untrained (except in some circumstances - See below, Section 1.2); one cannot take 10 or 20 on the skill, as it represents a contest of wills between two parties. Mercantile is a class skill for Bards and Rogues. This skill is distinguished from the Profession (Merchant) skill by the inclusion of a mechanic for individual bartering sessions.
 How is the Mercantile Skill Used?
When two characters (for example, a player character and a merchant) haggle over a sale or purchase price, they make opposed Mercantile skill checks. The results are interpreted as follows . . .
- Take the difference in the two results by calculating (Player Result - Merchant Result).
- Multiply the number from 1 by 0.01 (1%).
- If the player is buying an item, subtract the value from 2 from the number 1.00 (unity). If the player is selling an item, add the value from 2 to the number 1.00 (unity).
- Multiply the value from 3 by 1/2 of the item's listed market price.
How does one interpret the above procedure? It is much simpler than it appears! Basically, if a player wins the opposed Mercantile check, (s)he benefits, either by selling an item for more than it would otherwise sell, or by buying it for less than it would otherwise be purchased.
Devis the bard wishes to sell his trusty masterwork rapier so that he can buy a potion of cure moderate wounds. The market price of a masterwork rapier is 320 gp. Devis is not trained in Mercantile (silly Bard!); however, he has a 16 (+3) Charisma. Devis uses the Mercantile skill untrained. Devis' player roles a 15, for a total Mercantile check result of 18 (15 + 3). The merchant, who has 4 ranks in Mercantile and 14 (+2) Charisma, roles 10, for a total Mercantile check result of 16 (10 + 4 + 2). Lucky Devis! The difference between the two results is 2. Therefore the sale price of the item increases by 2%, from 160 gp (half of 320 gp) to 163 gp 2 sp. Use of the Mercantile skill has earned Devis 3 gp 2 sp - enough for a good meal and a pint of ale!
 When is the Mercantile Skill Used?
The Mercantile skill is applied whenever characters are buying or selling equipment. All items (weapons, armor, equipment) are subject to haggling; however, note that because trade goods (art and gems) are treated as currency, they are not subject to the Mercantile skill. The Mercantile skill may also be used as a Profession skill. A Mercantile check is made to determine the amount of money earned in a week of dedicated work - in this case, the work is buying low and selling high. In a week of work, a character earns a number of gp equal to one-half of his/her check result. Remember when making the skill check that Mercantile is a Charisma-based skill, even in this application. Using Mercantile in this manner requires at least one rank in the skill; when used untrained, haggling earns a character 1 sp per week.
 Thoughts and Errata
Why have a Mercantile skill? I find that it makes urban encounters more interesting; the merchants are real people with whom the PCs must interact, not simply lists of items and their market prices. The question arises: how many times does one apply the Mercantile skill in a single bartering session? I leave this to the discretion of the DM. Personally, I ask my players to roll once for each lot of identical items. Therefore, if a player is selling a halberd, a potion of delay poison and three identical battleaxes (the lattermost looted from Gnolls), (s)he would make 3 Mercantile skill checks: one for the halberd, one for the potion of delay poison, and one for the lot of three battleaxes. One can greatly simplify the procedure of selling items with the Mercantile skill by making a simple Excel spreadsheet.