Depends on the maker of your guns. For American made guns I have had good luck & been satisfied with Reddings Auction Service in Gettysburg PA. Last time they charged me 15% (this may depend on what you have, for a collection of junk or Marlin bolt shotguns or Mossburg 500's I am sure they will need more) & have no buyers premium.

They sold my Win 21, serial #2 a few years ago.