I like the 6 hour window idea, and then being locked in to the auction.
Alternatively if we don't go the route of placing accountability on bidders, then perhaps the withdraw bid option can have a cooldown period, so you can withdraw your bid only once every 6 hours from the entire auction house. This would benefit keeping harassment to a minimum?
The idea of being locked in after 6 hours seems more fair to all parties.

An idea about the previous bidder not being able to afford the auction anymore is that either everyone's, or the last two people's PC stays taken away until the auction ends, rather then until they're outbid.
Or after you place a bid, the previous bidder's PC will stay taken away for the duration of 6 hours. After 6 hours the next bid becomes obsolete, and thw previous person's PC is returned to them.

Worth mentioning but it should probably only be possible to withdraw your last bid. If you bid on one auctions 7 times, you probably shouldn't be able to withdraw any bids except your very last one (provided there is nobody above you)