Merchant Dispositions using Reaction Rolls
Merchants are one of the few people who have liquid coin to buy treasures dredged from dungeons and may have the inclination to do so (Nobles would rather claim it as theirs to begin with and the Church will happily accept donations but maintains paying for treasures is sinful and base). Even then, the goods must be either something the merchant feels they can resell at profit or want personally as a status symbol. So they will usually offer less than its value.
The following is a procedure for dealing with merchants with a modified reaction roll table. Make a 2d6 roll for a given merchant. This can be modified by character bonuses or circumstances as you would any reaction roll. Then consult the following for the result:
2 or less: Offers 50% of value for the goods, but hires thugs to rob you and recoup their coin.
3-5: Offers insultingly low purchase prices (10% of value). Reaction rolls to haggle at -1.
6-8: Offers 50% of value.
9-11: Offers 50% of value. Reaction rolls to haggle at +1.
12 or more: Offers 75% of value, reaction rolls to haggle at +2.
Keep the rolled disposition for a known merchant the party returns to, unless changes to circumstances would mean a change in attitude.
Haggling
You can haggle with a merchant by making another reaction roll once. The end result assumes you’ve gone back and forth and exhausted negotiations. Any good arguments for the player or trade skills should factor into this roll as bonuses (or maluses if they are poor hagglers).
2 or less: They refuse to budge on their offer.
3-5: They will pay 5% higher than initial offer.
6-8: They will pay 10% higher than initial offer.
9-11: They will pay 15% higher than initial offer.
12 or more: They want this for themselves, and will pay full value. There is a 1-in-6 chance they pay 25% more than its value out of desire to own it.