The only stuff I've bought on eBay are things cheap enough that I could take a risk without driving miles to see them first. It might be different if it were a fixed price, but if an auction and I have invested time and money getting the vehicle inspected I suspect I would be more likely to keep on bidding beyond the price where I would stop had I not invested that effort in.
Though I suppose the solution would be to decide on the absolute highest amount you will pay before you start bidding and make a resolution not to bid if it has gone over that, and also maybe leave it until the last 30 seconds to bid, and bid that final amount - so there is no room for you to be tempted to counter bid if someone has put in a higher proxy bid. Of course you are still risking spending time and money inspecting something that you decide is perfect for you only for someone else to outbid you.
Steve.