The best way to do this would be to set the seller up with a CIM (Customer Information Manager) profile and billing profile. This will allow you to charge him for any amount at any time without him having to re-enter his credit card information. You can do the same with the seller, if you don't mind having him enter his credit card information up front when he bids, or you can use AIM for a one-time charge every time he wins a bid.
As for tracking transaction information - you can get a lot of specifics on transactions by using the Transaction Details API in combination with transaction ID's. However, how the transactions relate to each other is something you're going to have to work out on your end - at minimum I'd recommend storing the transaction amount, transaction date, and transaction ID every time you make a charge, and if you're charging against a CIM profile, the profile ID. This will of course require database experience and a knowledge of how the auction system works.
If you have no idea how to do any of this, you should probably hire a programmer who's familiar with auction sites or a premade auction system. Authorize.net isn't that hard to learn (as long as people are responding promptly to questions here...), but auctions themselves can be quite complex.
