Cobb can unmarry your accessport for about $300, you need to ship the accessport back and they will reset the AP so it can be married to another car. However, this leaves a car with a "locked" ECU that cannot be read out by or re-written SSM or ECUflash.
I would suggest that you go back to the dealer and tell them about it to unmarry your AP.
Go to the dealer and tell them that yu mistakenly left the car's ECU tuned for some modifications that you removed prior to selling it to them. The car may run OK for a while but it's basically a ticking time bomb. It would be a huge disservice to pass the car on to a customer in its current state. Apologies profusely for the oversight and generously volunteer to return the car to stock before it's too late.
And remember, keep apologizing to them for putting them into this awkward position.