GunsAmerica and GunBroker are excellent sources for guns, and are great for finding good deals (saving $$$ on used guns) and particular models that are not always found at your local gun shops/regional gunshows.
They go to great lengths to make sure all transactions are genuine and completed to the satisfaction of both parties. They also emphasize adhering to federal law when completing transactions, but still leave some discretion to the sellers and buyers.
I have sold a few guns on Guns America, no problems.
I do ALOT of buying, and some selling, on Gunbroker.com. Again, no problems, and have saved hundreds of dollars on the pistols I search out on their site.
Of course, there are no 100% guarantees... so as with anything of this nature... "buyer beware".