The Alloy project began when I needed to integrate with eBay's XML Trading API and there were very few resources online that showed you how. Alloy will show how to list an item for sale on eBay using various programming languages. The first language being PHP and the next being Ruby.
Update: Checkout Cody Fauser's Ruby eBay project I forked. It's awesome, makes every possible request and response call easy.
Other sales channels in the works are Amazon, Yahoo, Google Base and more.
Check out the Alloy Wiki for tutorials and instructions: http://wiki.github.com/nuandy/alloy/