In any case if you reset the phone any software for trace will be deleted.
The real problem is another, if the old owner remembers the imei code he can block the phone, but if it is so I think the phone now should be already lock.
Just tell the seller that you are going to check wherever its stolen or not as a precaution as you bought a stolen phone before. If he says go ahead, then still do it, but you are 99% all clear, if he legs it, well, you won't have to bother checking it
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