If you still have the phone activated and all, can you do ##786# and let us know if under "Refurbishment Status" and "Refurbishment Date" says anything? And when was the "Original Activation Date" located at the bottom.
I also bought my Diamond off Ebay but received it directly from the seller and for me, those values are:
Refurb Status: No
Refurb Date: <no data>
Original Activation Date: 12/1/2008 (this is when I activated the phone).
I must jump in to say if the phone ROM was ever flashed, these values are erased. I know, I flashed my refurb Touch then flashed back to Sprint and it said it was not refurb and active on that day only :-\.
I like eBay for cashback but now I see they reduced it to 20% so since I want to smite Microsoft, I may either abuse them by not using their services and suggesting others never use virtually anything of theirs or abuse them in other way I probably would not buy a phone off eBay except in very particular circumstances and if I had problems, Paypal Dispute Resolution Center almost always sides with the buyer. If you can, alert your state attorney general about insurance fraud these sellers are committing; heck even alerting eBay may be a good thing.