Just to be clear for people researching old threads for this, sorry to resurrect, but the real story is that the carriers themselves don't blacklist for non-payment. What does happen is that the insurance company Asurion (and T-Mobile has a similar named one like Assurant) does blacklist if it's reported stolen or lost and the person gets another phone from an insurance claim.
Then when the new owner of the old phone finds out the phone has been shut off, the carriers don't like to tell people that their phone has been reported lost or stolen like you're accusing a customer of being a thief, so they will tell people that the previous owner did not pay. They also like perpetuating this myth to help discourage people from selling phones. But in the end they don't care as far as their books are concerned, they sell the debt off to a collection agency and on average end up getting something. Who it does matter to, is the insurance company.
Now because of this, keep in mind that this can happen EVEN with a paid off device. Basically whether the device has payments or not doesn't even matter in a transaction. What matters is whether that seller will turn around and do an insurance claim. That's why these sites like Swappa are safer, is because people are a lot less likely to do an insurance claim if some site has their info and can come back to them if the phone stops working.
