http://developer.ebay.com/developercenter/php/
PHP sample code to interface with the eBay API. Example code includes getting items, getting search results, and so on. Full documentation is available for all API calls.
http://www.channeladvisor.com/
ChannelAdvisor provide an eBay checkout solution amongst other things. LuzernTech use ChannelAdvisor to power their eBay checkout.