[The Donster]

You guys seem to have strange ideas regarding what "market price" is.
Market price is not a single fixed number. It's a range within which both buyer and seller can agree on a price. On ebay, prices are either agreed on by direct negotiation or by auction. An auction by definition does not have a fixed price.
Shill bidding cheats buyers by pushing them toward the higher end of what they are willing to pay. Let's say there's a car for sale and I'm willing to pay a max of $30K. The reserve price is $27K. The market value of the car is therefore $27-30K. There are two other bidders, a legit bidder and a shill. The other legit bidder is willing to pay a max. of $28K, so I should get the car for anything over $28K. However the shill comes in and bids $29K, forcing me to pay another $1000 for the car, without exceeding my max bid. I have just been cheated out of $1000, and don't even realize it.
This is how shill bidding works, and this is why it's illegal. It's also why shill bidding is rampant on ebay- it's extremely easy to have one of your friends place shill bids or else place them yourself using a different account and IP address. It's almost impossible to detect.