I agree with meeting them at vzw. Heard some guy bought a tour off of craigslist and checked the esn. It came up as clear but after the guy bought it, the seller claimed it stolen.
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Offer to meet them at the Verizon store. While there activate and then pay them.
You should take this advice since most people with bad ESN wont meet at a Verizon store. Whatever you decided to do, if you buy a phone used and not through Verizon then they technically wont honor any type of warranty or insurance.