At the risk of sounding like a non-response... it's whatever the market will bear. And when the market doesn't bear what a greedy seller would like to obtain, he uses one of his other (many) IDs to bid up the price of the item.
I've heard that the user in question does that and sometimes does it in conjunction with another Ebayer to artificially inflate the prices of antiques every now and again.
I personally can't substantiate it, but I assure you that I have it on good authority.