I have bought from both but only once from GunsAmerica and did not like them. I now only buy/sell on GunBroker but be aware that their "Buyer's Protection" (and I think it is limited to $300) means nothing. I once bought a Colt Python from Gunbroker and it would not fire on the second round though it looked like new. Gunbroker said that since I fired it, just one time, they could not offer their Buyer's Protection. Of course, I would never know there was a problem if I did not fire it. It had to be sent back to Colt who fixed it for a very expensive cost.
I now look at both feedback and only will buy if a credit card is part of the deal.