First, look for the Vendor button in the backpack interface and click it to open a stall. Economics dictates that competitive prices will help sales, so make sure when you set prices you don't get to greedy. Gold can be hard to come by, so players are always bargain hunting. Give them what they want by pricing a particular item a little below average and watch as sales soar. Also, don't think that common low level armor or weapons will sell like hotcakes, more often than not they will sit on the shelves. Try to find rare and valuable items, high level weapons and armor and things that you can't find just anywhere. Also, to top things off, set a clever shop name to attract attention.
If you're the one doing the shopping, then make sure you know what the average price for an item is, and make sure you know what you’re looking for. Also, checking the item description and price before clicking buy can be a big help in preventing suspicious transactions.