Bargaining is a skill that determines the rate at which you buy and sell at the shop off the "value" price for an item. It is trained by buying or selling things. And no, using the "I didn't mean to sell that" box to retrieve an item you just sold in order to sell it again doesn't train the skill. I tried.

At 1000 points, you will sell for 75% of value and buy at 75% of value