The market price, market value, or usual price of an item represents the price an item will typically sell for. The market price for a given item is a fixed quantity; when a customer comes in and asks for an item, the market price and the difference between the price being offered and the market price are displayed on the right side of the box. In the example below, you can see that the market price for a lucky amulet is 635 gold, while the customer is offering 995. Hence, the difference of +360 is displayed as the profit.
When selling to a customer, you obviously want the offer to be as much above market price as possible, and when buying from a customer, you want the offer to be as much below market price as possible. In the early game, when your market affinity is low, you will find that most offers are relatively unfavorable compared to market price, so you will often have to sell below market price in order to get a sale. Conversely, when your market affinity gets very high, most customers will be offering to pay well above market price and you may not want to sell unless an offer is 25% or even better above market price.
Note that the "market" is entirely theoretical -- there is nowhere you can actually buy or sell items at market price.
The success rate of haggling is determined by the price relative to market price. Attempting to haggle a customer offering to pay much less than market price is very likely to be successful, while attempting to haggle an offer already above market price is very likely to fail.
Like most monetary quantities in the game, the market price is rounded when it gets larger than 1000. To see the exact value, mouse over the number displayed, as in the example below.