I have purchased 2 pistols and sold 2 on GB without problems. Thehighroad, glocktalk, and hkpro are other good places depending on what you are looking for. Most precautions that one would take on ebay apply at gunbroker. The important thing is to do your homework and know what you are buying. Then ask via email the exact condition of the firearm and any other pertinent questions like build year if you know that there were changes during production. I.E., I was looking for an early year HK USP .45, since they did not have the internal lock. I found one in excellent condition, and the trigger is noticeably better than the later ones that I have shot. It was about 98% and I would not sell it for the price of a new one.
Most people seem to be inclined towards honesty due to the rating system, although there is always the exception. You ask the questions in an attempt to weed out those exceptions.
I also have a theory that most people don't really shoot their guns that often. When I think about the folks that I know who shoot a lot with all of guns that they own, then add all of the guns bought and sold by everyone else, the numbers would add to huge amounts of range time. And how many ranges are there and how often are they used? I think that the average gun owner buys some protection, maybe goes to the range a few times a year, and feels like that is enough. IMHO.
Which is a very long winded way of saying that I feel pretty confident recommending gunbroker.com, providing one is are willing to do a little homework.