Over the past year I have bought at least 25 firearms off either Auctionarms.com, or Gunbrokers. With the exception of two, all were as advertised and fully met my expectations. Of the two that were not up to my specs, one was returned for a full refund, the other one I deceided to keep after the seller agreeded to return a portion of my money.
The key to buying, figure out what the gun is actually worth, keep a sharp eye on the auction, and wait until the last 30 seconds to place your bid. Place a "PROXY" bid to the full amount that you are willing to pay. You will still miss a lot of guns, but you will buy quite a few, and at the price you were willing to pay. I suggest you purchase a "Blue Book of Gun Values" and keep it at the computer. Don't bit until you have looked at the book.
The problem with most folks that buy at an auction is they are so eager to get the item that they overbid, don't get caught doing this. First, it will likely cause you to pay to much as you get wrapped up in the excitement of bidding, second you may find yoursellf saying, "I'll show this guy he can't out bid me." When this happens, you usually end up paying too much.
Gubbrokers and Auctionarms are safe, and there are good buys there from time to time, just be cautious.