Practically nothing, and it can prove expensive into the bargain. Some sellers start at one euro and then attempt to bid the desired minimum price up using a second account. Since most bids are in any case only placed in the final minutes, vigorous bidding days before the end of the auction fizzles out to no effect. If the seller ends up buying his own item, he must additionally bear the platform's selling fees, which quickly becomes expensive.
A variant serves to spy out other bidders' maximum bids: the seller keeps bidding under a second account until he has outbid the previous highest bidder, and thereby knows that bidder's maximum bid. He then cancels his own bid and places a new one exactly one euro below it. This approach, too, usually comes to nothing, because the decisive bids only arrive shortly before the end of the auction.