They are worth what someone is willing to pay. I've always listed on the EE at what I considered to be fair prices. Some go in minutes or hours, some take days or weeks (with occasional price drops). I've made offers that didn't get answered or flatly refused. Other offers quickly accepted or counter offered. Dealing this way, I believe always has the item sold at market value. A gun that is bumped to the top many times at the same price is obviously over priced. List yours at the high end of your expectations and adjust according to the response, or lack of response to your ad.