It depends on the guns. I like to go on a forum dedicated to that particular gun. For example, I like to sell my Colts on coltforum or my Smiths on smithanwessonforum. There is rimfirecentral, shotgunworld, etc...
By the way, to look for guns to buy, you can't go wrong with elitedealseeker.com
Take some to the Indy 1500 this weekend. I mean, the dealer won't give you much for them, but private sellers are generally pretty good to deal with. It's a good audience.