A bidder is making bids, he's not the owner of the product, so I think you mean auctioners are copying their auctions to other sites. That obviously has the prerequisite it's not the auction of a single item, but since eBay rather has become a shopping mile anyway, that's perhaps not the problem.
I don't know how to exactly answer your question about API vs dom parser, as they don't compare at all. The intention of any API (application programming interface) is to have an open interface where eBay itself interfaces to it's data, but also further applications can extend eBay.
The API is a programming interface answering your questions via methods foreseen to give the detail infos you need in a better quality than you can scrape off, but the approach totally differs from scraping information. With allowance of users, the API even enables starting auctions, changing auctions, everything you might do on the behalf of the eBay user. The API methods are official, directly "wired" to eBays database, so they give concrete infos as eBay stores in their backend(s) to put together the auctions HTML themselves.
Via the API you go directly at what you need (and what eBay offers). I don't think you can get any detail from scraping an auctions HTML, which you can't get from the API. The limit of 500 will hit you with a few users, so you would need to make your application apply to the rules specified, for example about OWASP safety guidance. It's not at all a bad idea to apply these rules to any web application anyway (
Bye, Olaf.