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For the devices that are tied to a payment plan on AT&T I'd recommend selling locally through Facebook or Craigslist and meeting the buyer at an AT&T store. Then you could use the money from the sale towards paying off the device and the person who's buying it from you can be assured that it's definitely paid off. I think if you're upfront with the buyer they wouldn't have any issues with doing this.

I'd say Craiglis and keep the following in mind:
  1. Meet only in public places (malls work best)
  2. Accept cold hard cash only
  3. Patience!
Both of these suggestions are great!

If I were in your shoes I do craigslist and / or facebook marketplace. Meet at the AT&T store and as soon as the buyer hands the cash over hand it to the ATT rep to pay off the balance(s)..

This will give the buyer piece of mind that the device is truly paid off. As a bonus you are meeting in a public place with cameras so anyone with nefarious intentions will pass up the offer to meet there.

There are plenty of horror stories about deals gone bad on craigslist (robberies, counterfeit money ect.) so if someone acts fishy over meeting at ATT then I'd move on to a different buyer. Anyone legit would be happy that you want to meet at the store and instantly pay off the remaining balance due on the device.

Good luck with the sales!
 
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