Here's my take, having bought and sold on all three.
GunBroker: Still the largest venue, with the broadest reach. You can find bargains once in a while, but for double guns specifically, it's usually just the same small number of dealers, with guns priced for 'lazy dreamers.' What I mean by that is, prices that will appeal to only the least price sensitive buyer. Because they are so big, navigating their help function feels a little detached.
GunsAmerica: I think private sellers and dealers whose primary business is NOT on the internet, prefer this format. I've never liked the look and feel of the site, and the downside of some private sellers is they are clueless about details, don't want to ship, etc, etc. But that's also how you get bargains, and I have found more here than any other site...hands down. I'm talking about ALL guns, not doubles specifically (but there have been some...I positively STOLE a gorgeous L.C. Smith Ideal a few years back). I have not yet interacted with staff, or can't recall.
GunsInt'l: Nice format, and the open contact info makes it the most conducive to getting to negotiation. I think from a quality perspective there is as much or more than GB. And the universe of buyers seems to be more specific than the others. I say that as a GOOD thing, because you tend to get fewer but better quality (i.e. higher liklihood) inquiries as a seller. The staff are helpful and responsive.
Nutshell:
For BUYING: GI and GA...because of prices and negotiation ability.
For SELLING: GI and GB...for different reasons. GI for ease and format, GB because of market reach and sporadic "have to have it" bidders.
NDG
