I don't think its worth it to buy an unlimited account from another user for hundreds of dollars. Not only is it complicated to transfer, but AT&T could cap or end those old unlimited plans at any time. They did with the unlimited iPhone plans and I expect they will eventually do it for the iPad plans as well -- especially if they add the hotspot feature Verizon now has.
What do you mean they capped it? I just checked my AT&T bill online and I'm still on the iPhone unlimited plan.