Well if you want to buy guns that are not registered to you, don't buy them from a store. You still have to pass a NICS check here.
The site I use the most is
Sometimes they are over priced, sometimes there are good deals. I've bought all of my firearms from it except my Taurus, which I got at Buds.
Also don't forget about centerfire, one of the best imo, for MBRs.