The eBay developer site is at developer.ebay.com. From there, you can read the documentation (available in PDF) for SOAP, XML and REST.
You can read more about SOAP at www.w3.org/TR/soap12-part1 and about XML at www.w3.org/XML.
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.
