On ebay it's a serious offense and they suspend or terminate the accounts of shill bidders. A while back I reported someone that I suspected of shill bidding and they terminated the accounts of both the seller and the bidder. (The bidder was bidding on many auctions of just one seller. He didn't buy from anyone else. That could legitimately happen but there were some other things that made me suspicious, and I just figured I would let ebay decide.) I know of another person (from an ebay board) who bid on her daughter's item because the item had been damaged during the auction and it was close to the end and they couldn't cancel it. Ebay suspended both of them. They figured it out from the IP adresses -- evidently they monitor them.
But when it comes to Goodwill, are the shill bidders getting money in their pockets for doing this? Even though the dollar amounts may be low it is definitely worth prosecuting because they're scamming *everyone*. Petty shoplifters get prosecuted -- why not Goodwill. I've never bid on any GW items but I might in the future so thanks for the heads up.