There are 2 ways to sell items - right click on it and select sell, or take it to the Market and put it up for auction.
Note that the Market is NOT the same as the Auction house. The Auction house is used by the system to sell new, high end gear to adventurers, whereas the market enables adventurers to sell items to each other.
- Right click on the item and select Sell. Seems to work anywhere - you don't have to be in an appropriate shop.
- You get a fixed price (which you don't find out until after you've sold the item) and the item is destroyed.
- Go to a town and find the Market. You'll see a vast list of items offered for sale. You can use the filters to reduce the list to a more realistic number.
- To offer an item for sale, go to the Sell tab and you'll be able to drag the items you want to sell over to it. Up to you whether or not you want to tweak the suggested prices.
- Note that you cannot market bound items. Once you put on an item it becomes bound to you and cannot be sold at the market auctions.
- The item will be displayed in the listings (with some lag) and will have two prices - the current bid (if there's no name associated with it, then it's you initial asking price) and a 'buy it now' price.
- Note that you will have to pay a comission price to put the item up for sale.
- You get notified of the outcome of the auction by mail - an email with either the item or the gold for it attached. If an item doesn't sell at the market, you can sell it directly from your inbox.
Deciding where to sell an item isn't to hard. The only stuff that should go to the market is the valuable stuff - +4 enchantment or better, significant stat bonuses for its level, pre-existing sockets or gems.
As an alternative to selling items, you can dismantle them for magic debries that you can use for trade or to improve the changes of a refinement working.