This is a reverse auction, to win the auction all you need to do is place the lowest unique bid.

For example:

Customer A places a bid at 5p
Customer A places a bid at 6p
Customer A places a bid at 8p
Customer A places a bid at 9p
Customer B places a bid at 4p
Customer C places a bid at 5p
Customer D places a bid at 8p
Customer E places a bid at 8p
Customer F places a bid at £1
Etc
After the auction has closed customer B is the winner. As their bid is the lowest unique amount.