Bought an iphone 5 on eBay that was financed?

I bought an iphone 5 on eBay recently and tried to use but when I checked to see if the IMEI # was working, Tmobile tells me that the phone is currently on a financed plan and I guess the previous owner didn't fully pay for it before selling it. Is there any way I can go about getting this fixed or should I just return it?

Return the iPhone to the previous owner