Check the "sold listing" gives you an idea of what to expect for buy it now and auction style - also watch those with similar spec and see what they close at.
As a starting point, you can check Mac2Sell, eBay (including completed sales), and Amazon for prices of similar models, to get an idea of what a reasonable price may be. Then adjust the price to account for condition, configuration, remaining AppleCare coverage (if any), etc.