The eBay developer site is at developer.ebay.com. From there, you can read the documentation (available in PDF) for SOAP, XML and REST.
My examples this month were in Ruby, and you can read more about it at www.ruby-lang.org. There are SOAP libraries for all programming languages that I use on a regular basis, including Perl, Python, PHP and Java. REST doesn't require a specific library, but uses the HTTP library that comes with your favorite language.
Finally, I recently enjoyed reading The Perfect Store: Inside eBay by Adam Cohen. It is slightly dated, but gives many insights into the 900-pound gorilla of e-commerce. If you ever have used eBay, you might well be fascinated to read what it was like in the early days.