You could put an ad in the local Trading Post that says "I have guns, Please come steal from me"
Sorry- been dealing with teenaged grandkids this afternoon. I'll be OK when I can hang out with adults for a while.
The three largest sites I know are Gunbroker, Auction Arms, and GunsAmerica in that order. DO NOT try to sell all in one action- you limit your market. I may want one, but not the other 12 (unless they are connected to each other in some way) If these are not muzzleloaders or pre 1899, they will need to ship TO a FFL if crossing a state line. You can also talk to a local dealer-for a a consignment sale. You CAN ship rifle or shotgun by US Mail, Be SURE it is unloaded. I mean DAMN sure.
If you do on line sale, state in the ad that you are NOT a FFL, and receiving FFL must be willing to accept shipment from a non-FFL (it is legal, some will not do it) Send a copy of your driver's license with the gun so he can log it in to his books. Good description, good pictures, do not do a hidden reserve auction (many of us skip those entirely). Set absolute minimum you would take, and start the auction.