There is a thread called Stock. In there you will find a list of every item sold in this shop, and how much it costs. When you've decided what you want to purchase, merely reply to the Stock topic. It makes things easier for the administration to manage, and otherwise there would be hundreds of topics floating around the store.

The Item Store is also where you can pawn off your goods, and spare equipment. If you wish to sell something to the store, or try to trade it off to another player in this realm, then you can start a post and name it accordingly. "Selling: so and so" or "Trading: such and such", something so the admins can tell what's being sold to the store and what's being traded, so we don't get confused between the two. We'll later have store owners who will moderate their store and that's the person you'll deal with when selling your stuff back to the shop.

To sell back an item, you simply say what you're selling back, and state an asking price. You can deal and haggle from there for something better.