I don't really know either... but this is what the wiki knows:
Selling
You can also sell things to traders. Bones, corpses, body parts and rocks are not valuable, no matter how attached you are to a particularly aerodynamic kobold head. Small creatures discovered while Looking Carefully may be worth a small amount of money. In order to sell or buy items, stand adjacent to the shopkeeper in his store, and konverse with the shopkeeper. Select "Trade" and press enter to open the trade window.
Select each non-worthless item you wish to sell, and then set a price using the following formatVerify:
* a asking for 9000☼
* s +100☼
* d +10☼
* f +1☼
* g reset to 0☼
* h -1☼ (offering)
* j -10☼
* k -100☼
* l offer 9000☼
The use of these keys may seem non-intuitive, and this is further complicated by the limit on your available offers by your current financial health.
Shopkeepers are used to adventurers with inflated ideas about the value of their goods, so it may be simplest to ask for 9000☼ for your goods, or offer 1☼ for theirs and suggest a trade. The shopkeeper will counteroffer with the actual value of the goods, and will be quite delighted to accept a trade at the price they've just quoted to you. You can then purchase things with your store credit. After the trade sessions, the balance of your coins will appear on a small table next to a chest.
Source:
http://df.magmawiki.com/index.php/40d:Adventurer_mode#Trading#ixzz0xELeIj4t