What brand gun is it? There are lots of sites out there for specific brands of guns. I know for sure there is a Colt site and a Smith & Wesson site, I believe there is even a Ruger site. I would suggest going brand specific, because a lot of the collectors tend to hang on the brand specific sites, and since they are collectors, they usually have a very good knowledge of what a particular guns value might be. If they don't know off hand they will have access to the blue book of gun values.