Considering that up until now, the eBay API has not been that easy to get (it took me 3 days to sign-up for it when I built aytozon.com), that's very impressive. (BTW, if the eBay API is getting 2B calls/month, how likely is it that Google search is getting more than 2B US searches/month?)
There are over 90 calls in the eBay API, accessible with SOAP. There's also some REST calls, which I haven't had much success with. You can download the 1298 page PDF manual here. Go crazy.