>I am writing an appliction that tracks the sale of collectables on ebay (just ebay for now, perhaps other online auction sites in addition later).
In addition to the other suggestions, consider signing up for the eBay dev kit. It's currently in beta, but it's something to consider for the future. Details should be on their web site. I signed up for the beta, but haven't heard anything back yet, so I can't comment.
Mike Stewart