Tektek.org is a good site, that will tell you what the current prices are for various items. Generally, anything you buy in stores you can sell on the market, so long as you price it less then the store. It's good to look at the current market value before you set your price so you can determine if you are asking to little or to much.

You can also look to see what they are worth, then make a thread in here asking for about that much, or accepting offers.