A system similar to this has been planned for months and is awaiting completion. It will allow players to sell items at store tab prices to an external buyer (ie not anyone on the server, removing the items from circulation), allowing players to get rid of items they don't want with only a small amount of hassle and without losing too much money. More information relating to this will be disclosed at a later date.

I think the price should be set below store price.

(And I hate your backwards text box...)
Why? Generally you don't buy items for store price, you pay store + whatever tax the merchant wants.

Well, because you can use your system as a "quick sale" feature, and for quick sales you usually lower the price. Or you can put some effort into selling it and get normal or above price.