From everything I've read, it does marry the unit to the particular car. I would assume this is done because once the ECU is reprogrammed, you can unplug the AP from the car. What would prevent someone from buying an AP then charging friends/etc $50 or so to reprogram ECU after ECU. From the SU page for the AP:
"
You Purchased an AccessPORT from Someone on eBay and it is Married
- Assuming you are not able to contact the seller, you have to send us the AccessPORT and we un-marry it. However, Cobb will charge you for the license ($395) since the old one is still out there making another car super fast."
So, buying a used one isn't a problem, but I would buy it from someone you know or with some mechanism for dispute/return/refund should the person not have 'unmarried' the unit before selling it.