Does Gunbroker allow for you to see other's bid histories? It'd be comically easy to spot shill accounts in that case ("gee, this same guy with a very low feedback score has been around for a year and only bids on this other guy's stuff")
There is a contingent of GB-trollers out there who thrive (if you can call it that) on 'sniping' random strangers of good deals. It'd be like some loser cutting in line at the grocery store, grabbing your pack of steaks, and saying he'll offer 1$ more than what you were about to pay. Now, personally, I don't get it, since the time you spend working this ridiculous gambit is considerable for the payoff, but I guess it's a way to spend the time for those poor souls who can't/won't do much else. And each auction win is as much a 'victory' for these slobs as is a skilled gamer pwning a random noob in some online game; some small point of light in their dim day.
There are folks who sort by "soonest ending/no reserve", and scroll for auctions with a single bid, and are more or less guaranteed that the price for what they are looking at will be at or below "fair." If they make a nominally higher bid, they are almost certain to be capable of reselling the item at a slightly higher price point.
They aren't official shill bidders, since they rarely bid on a 'bad' deal to drive the price, and do seek to win, but they are like "value day-traders" in the stock market that always look to 'correct' those nagging price discrepancies that make a share worthy of serious investment interest. Part and parcel of capitalism.
I have, however, found a trick to combat this behavior, that has had about a 30% success rate (almost
every bid I made before doing so would be contested at least once). It's as stupid as it is seemingly effective. You see, when you sort by 'soonest ending' and look for bid activity, a single bid will look the most enticing to a sniper (or shell bidder, for that matter). So make your first bid a few bids above minimum, and enter a few consecutive bids; they will show in a search, and appear as legitimate competition to anyone not looking at the actual price or bid history. They see a number higher than "1" in the bids column, assume there's already competition/sniping going on that they don't want to compete with, and move on. About 1/3 of my bids go uncontested since I've started doing this.
I figure it's as much for my benefit as for the addicted loser who won't be borrowing money to take home some random gun-widget he had no interest in to begin with
TCB