EBay! Hands down.
Ive sold most of mine there, and made good money doing so. Cleaned out all my "holster" boxes, drawers, etc. too. Old web gear, mag pouches, chest rigs, etc, are also good sellers.
Used night sights and gun parts are even better. What M1 Garand parts bring these days is unbelievable!
Ive listed a few things, including holsters, on a couple of different gun forums, and only ever sold one item. Did have a couple of "Ill take its", that drug on and never came to fruition though.
Ive sold some things on Gun Broker as well. The auction sites seem to be the best place, and you dont have to wait to long. Most of what Ive sold went on the first listing.