Just imagine if you had a gun or pawn business or had a long time interest in gun collecting. People would either be bringing guns directly to you to trade in or to help them liquidate, or you would be attending live auctions where there are more of them for sale.
Live firearm and/or estate auctions are fairly prevalent around the country and some do offer live online bidding during the auction.
And in many places, the guns that police departments confiscate are auctioned off on a regular basis. For instance, the police will sometimes confiscate entire gun collections from people who have been arrested and/or convicted of certain crimes.