Ive been an eBay developer since it started about a year ago. The API is a fantastic bit of kit.. what does it do? pretty much everything, well it does do everything actually
You can if you want create a whole replica of eBay from it.. that includes everything, validating users, adding items blah blah blah...
Ive had a developers token so ive never paid anything, but for $250 its well worth it if you can find a use for it.
For your $250 you will only get access no scripts or software.