I have sold and bought iPad unlimited data and have had no problems. (Sold one and bought 2).
You can't sell your unlimited plan. Yes the unlimited plan is tied to the SIM card but the SIM card is tied to you. If you sell the SIM card and even walk the new owner of that SIM card into an AT&T service center they will not transfer the account to another name. So be super careful if you sell your SIM card because you maybe paying for someone else's unlimited data plan if you don't cancel it prior to you selling the SIM card. This is in the terms and conditions page of the unlimited plan. The plan stops when the account holder name changes. End of story. So those ebay SIMS are a scam.
Buyer beware!!!!
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