If you sell a handgun, you simply find a local FFL to ship. IIRC, neither UPS nor FEDEX allows private citizens to ship handguns through them anymore. My FFL charges $40 to ship and the buyer pays those fees.
Otherwise, you can join any one of a dozen gun forums and post your guns through their classifieds. Then follow the above protocol.
Thanks. Things used to be simpler when i grew up. Now, I’m just a cranky anachronism.