When the Auctioneer puts in a vendor bid STOP BIDDING because you are only bidding against yourself.
Although dummy bidding from a convenient tree is illegal it still happens so beware that some bids may not be true bids
As boatboy has said negotiate after if there are no real bids
There are lots more to buy so don't get carried away if it goes beyond your pre-set figure
It can sometimes help if you mention to your salesman before the auction gets under way that you don't think you can go over $??? (a figure which is far below what you can actually pay)to try and stop them forcing you up