Players will be able to pick a selling price of how much they are willing to take for whatever they are selling. Buyers and sellers have a sell history of each item to help them gauge the market value of what they want to buy or sell. Buyers don't get to see the price the seller is wanting for the item, though. They simply locate what they want to buy, and make an offer.

If what they offer is equal to or greater than what the seller wants for it, then the transaction goes through and the seller receives what the buyer offered (minus the appropriate fees, of course). This means that sellers who don't know the value of what they have have a greater chance of getting what they deserve for the item, no matter how low they personally value it.

Via Gamespot