Sprint bought my phone on Ebay

[Trahill]

Hi, I was wondering if someone could help shed some light on my situation. My family got new phones through contract renewal, but we're all fine with our old phones so I'm selling the new phones on Ebay. Well, one of the phones was just bought by Sprint Wireless. Why would they buy my phone? I didn't think I was doing anything wrong, but I'm not sure why Sprint would be buying a phone from me. Any help would be appreciated.