I do not think this is true, Tmobile and ATT will blacklist IMEI numbers for non-payment. And as of this year the two share the same blacklist. Down here in central texas this is a huge problem. Everyone can go into either store and pickup the latest samsung or apple product on the payment plan, stop paying and sell on craiglist.
I think that your best bet is to type up a bill of sale that explicitly states that the phone is paid in full, not stolen, etc.. Take a pic of the guys license, and license plate and nicely but firmly tell them something like this, "I am a busy guy but if you screw me over I will devote large amounts of time and money to find you and take you to small claims court where I will win"