I've posted a couple of firearms for sale in the classified sections of sites like this, plus
www.thehighroad.org,Glock Talk(
http://www.glocktalk.com/forumdisplay.php?s=&forumid=39), or elsewhere, and had pretty good results. Also bought a couple of things this way.
Works pretty good, and keeps the middle-man's cut out of the exchange.