The three you mentioned are the best. I'd go with AuctionArms and GunBroker over GunsAmerica, though. GunsAmerica is more of a gun sale rather than a gun auction. I just find better deals on the other two (possibly with the exception of magazines).
Anyway, I've done some searching, and the only other ones I've found generally have no more than 20 guns.
Finally, and you probably know this, TheFiringLine has some sales towards the bottom of the forums page.
Hope this helps!