Maybe contact the seller and see what he suggests about the HTC Warranty.
If the warranty is transferrable, you could have said you purchased the Diamond from a friend.
There are 2 questions that you need to know for sure >
1) Do you have to buy from an Authorized Dealer in order get the Manufacturer's Warranty (HTC)
2) Is a Warranty Transferrable or is it only valid to the Original Purcahaser (this way you know what to say in regards to getting your Diamond serviced under warranty)
The Diamond is warrantied for 1 Year from date of purchase, no matter what the above answers are.
Someone else may have more finite answers for you.
For future Ebay purchases, if the Manufacturer's Warranty is not valid on a new purchase, it would not be worth to purchase, imo.
Since the Ebay seller purchased the Diamond from Sprint, the Warranty should be Valid, but maybe to the original purchaser. But there should be a work around to get it repaired under warranty. But time is running out.
EDIT:
This may help regarding Ebay purchases and Manfacturer's warranties >
http://pages.ebay.com/help/warranty/buyer_faqs.html