Playing devil's advocate (not saying these are valid/ethical reasons or anything I myself would engage in, but you didn't specify):
1. You have been banned from the auction house in question and are unable to place a bid yourself.
2. The item being sold is yours and you want to increase the price on it, but the auction house has a policy against consignors bidding on their own items (i.e. shill bidding)
3. You have a friend who is also very interested in the item, and you want to own it but don't want your friend to see you bidding against him (in other words, you value your friendship more than the item, but only just barely). You'll just have to hide it whenever your friend comes over
