[Cicatrize]

First of all, to the people implying that buying a phone on eBay is a bad idea:

I have been buying as well as selling phones on eBay for the past 4 years. The only time anything bad ever happened was when I bought my first TP2, and it had nothing to do with a bad ESN. I returned the phone to the seller and he refused a refund, that's all. PayPal got that squared away quickly and got me my money back. One important thing about buying on eBay with an ESN based service like Sprint is that you make sure the seller guarantees clean ESN, and accepts returns. Almost all of them do both of those things. I got my second one off eBay as well and couldn't be happier. I didn't have to extend my contract, and the total price was cheaper than if I had (after taxes & shipping).

Now, on to your situation...

Does the seller you bought from accept returns on the item? If so, deactivate it and return it amicably. If the seller refuses your return of the item, file a dispute with PayPal (HOPEFULLY, you used PayPal. Buyer protection is nice to have). They will get your situation resolved.