Add credit system to an auction php script

Hi, I wist to start an Auction site with a user credit system where users can add credits to their accounts purchasing from Paypal or other Online payment sites. Then credits must be applied when a user creates a new auction. (For example: A user can make a $10 payment via paypal and once the payment is done the user account will be credited with 10/100 point or something like this. Once a use has credits in hes/her account can create new auction and charges an "X" amount of points taking off hes/her account) I don't have a specific Auction Script yet, but i am with open mind to get any advise from you what to use and what is easy to make it possible. I only need to have basic futures to auctions like email when user got over-bidden and thinks like that. If anyone has a better idea for this project i am more then happy to chat with you. Thank you.