Irunguns (site sponsor) offers the service to list on gunbroker. The gun needs to be importable to the US. Under 3" barrels I believe are no go and many weird exceptions and counterexceptions. Borderview, also a sponsor, offers a US export service, though unsure if tgey will list.
You can also get in touch with the Canadian auctions like Switzers, Landsborough, Meyers, GTA, Kidd - they have both US and European buyers. But again not everything is admissable. Many US buyers use Borderview to export their purchases to them.
I kinda wonder what the future of restricted gun auctions will be. Dealer only like in the UK?
What are you trying to sell? Very knowledgeable folks on here could likely tell you if it's importable to the US or other places.