Is there any way to assure that software purchases on eBay are original and not pirates?
Not beyond the usual things to do, with eBay. Check the seller's ratings, and comments written about him/her.
If you have a question, email the seller with it before you bid.
Read the fine print, in his ad.
... that sort of thing.
I tried buying a software suite through them once, the guy posted a picture of the retail package and what I recieved was a copy that he had made. I wasn't about to stick that in my computer.
I bet you could demand your money back, as I doubt eBay would allow piracy.
If nothing else, be sure to leave a comment about that seller, describing what happened.