AT&T originally offered unlimited on iPad, and then allowed grandfathering, which they also allowed to transfer from 3G to 4G for those customers. You can sometimes find resales on Craigslist and such, when people with grandfathered accounts will sell their unlimited data plans. I don't know what they're going for now, but at one point some people were selling them for about $1,000. Crazy, IMO. (The new owner essentially took over the account for $1k and took over monthly payments.)
People with grandfathered unlimited accounts can transfer/sell them by just turning over the SIM card and having the new owner sub out the credit card number.
AT&T hasn't been as aggressive with weeding out grandfathered accounts as Verizon has.
I'm going to hang onto my unlimited AT&T phone and iPad plans until I'm cut off, lol. Even if I didn't want them any more, they'd still be worth something to someone, even if I just transferred them.