I've sold a number of firearms on Gunbroker, never used GunsAmerica so can't comment there.

But I've had good results on Gunbroker, you get a huge audience and the listing doesn't cost you anything until you sell unless you add some of the special features, and even those are pretty cheap, maybe $3-4 total if you want a fancy listing and hit counter. Once it sells the final value fee isn't steep IMO, certainly a lot less than you'd pay for a consignment fee at a gunstore.

The other benefit is that you get a pretty good real market value for your goods. Lots of "it depends", but decent goods generally find a good price by auction and sometimes if you have a particularly nice or somewhat rare specimen you can get a lot more than you think. The other side of course is that your goods are listed alongside and competing with many others, but the very wide audience is the chief advantage.

Just take good pictures and describe it honestly without being too verbose and if someone wants it, they'll find it.

Gunnery, gunnery, gunnery.
Hit the target, all else is twaddle!