Transferring an existing iPad unlimited data plan is very easy. The seller just needs to update the account to change the email address and as long as you have a few days, I would suggest 4-7 days or more before the auto renewal charge to the credit card. It is very easy to update the credit card for the next month's charge.
I often see it done with a prepaid Visa card that has $30 loaded on it to protect both parties if there is any privacy concerns and the use of a free Gmail or Hotmail email account for the exchange.
Dave