When I see a shill at an in-person auction, I tend to bid him up and drop out just so he's left holding the bag. Once he knows what I'm doing, he may back off when I'm interested in something. I've had auctioneers attempt some fancy footwork to keep me from doing it, but they started the game and they should deal with the consequences. I generally, though, stay away from auctioneers who regularly use shills. Paying attention to bidder numbers helps. For example, one auction housenear here has all their shill numbers in the 60's, so you get to know what's going on if you're paying attention.
With on-line auctions, there's a wider potential audience and the need for shills should decrease, but as Ben eluded to, that's not always the case. Shills simply prostitute the notion of fair market value and arms-length transactions.
