in short, as previous poster mentioned, one can call the carrier, give them the MEID of the phone and the carrier will know right then if its stolen or not.
if the seller is not willing to offer this info, then leave. nothing you can do.
if you are suspicious of the phone and the seller, walk walk away.
many times, things are too good to be true.
a phone made for ATT, as an example, has somewhat different software than a phone made for verizon. subtle differences. check this thread for more info:
HTC One specifications and discussion (Keep thread on topic)
no need to continue this discussion as the ops concerns have been addressed and this thread will not continue to talk about using stolen devices in other parts of the country.