Hmm this is tricky.
Aside from making sure it's clean at a Verizon sale, another thing could be done is a typed out statement stating that you bought the galaxy s 3 from (name), with your and the sellers signature. It's more of a proof of purchase if you will.
That way if something like this happens, you'll have a "legal document" per say to go to.
Also if I'm not mistaken here, if the s3 is tied to an account, it can't be activated on another without the release of the original account owner.
With that being said, i wouldn't worry about it being reported as stolen without actual proof.
Oh and by the way, welcome to Android Forums!