As you can see on the eBay Developer Program Overview page they draw a clear distinction between the Shopping and Trading-focused APIs. The shopping APIs are designed for lightweight, read-only, search and buying REST-style functions that don’t require a token. In terms of performance these services are up to 16 times faster than older equivalent calls. They support a variety of data formats including XML, SOAP, Name Value, and JSON. The Trading APIs on the other hand are more sophisticated, data rich read-write APIs that require authentication. As they describe “For example, if you are building a browser-based search widget, you will want to explore the lightweight eBay Shopping API. If you are building a listing management application, you will want to explore our secure eBay Trading API.”
One note about the very interesting PlaceOffer API that will let developers integrate bidding into their applications is how eBay is quite explicit that “Sniping (scheduled bidding) through PlaceOffer API is strictly prohibited … Applications need to be verified before PlaceOffer API access in Production environment.”
The existing eBay APIs have been popular with mashup and ecommerce developers — there are currently 77 eBay mashups in our directory. Choose from Unwired Buyer to Mashed Tickets and the Flash-based interface of Cooqy.