Yes, ebay has shill bidders, but you can notice patterns for that kind of thing in the bidding history. With a private auction, where the seller is running the backend (the software), my question would be how do you know if anything odd is going on in the background.
Auction houses are a 3rd party, ebay is a 3rd party, but when a seller is running their own auctions....Like I said, not saying anyone running these auctions does this kind of thing, but the system itself seems a bit flawed, don't you think?