Interesting discussion going on over here where eBay would like you to pay them so you can build a product that will help market their product. Here's a thought: Use AppConnector to screen scrape eBay, then access it via your own web service.
While we are it - I have to respond to this comment by Jeffrey McManus (head of the eBay dev team):
scraping the site is a really bad idea, not just because it’s “against the rules” but for technical and stability reasons
This is not strictly true anymore - Screen Scraping (or Presentation Integration as we like to call it) has progressed a bit since the 80's. AppConnector is smart enough to detect changes like this. Besides, I really doubt that eBay changes the names of form elements every two weeks - it would mean changing their own systems to cope with it which would be cost prohibitive, not too mention a waste of time for the QA guys.