With my experience with Ebay auctions only, it is my understanding that it is legal for the seller of an item to place ONE bid on his own item during the course of the auction. This is allegedly to make up for the possibility of forgeting to place a reserve price, etc.
Keep in mind that if such a bid were to occur, it should only happen with the seller's user ID, and not some fake ID, or that of a family member or friend. That way it is clear to all participants of the auction what is going on.
This may very well be in the "rules" of ebay auctions, however I consider it to be dishonest, and will not place a bid on an item where I suspect the seller of bidding against me.
E-Bay rules MAY have changed on this policy, however.
This is yet another case where what is "legal" is not necessarilly what it "right".