The original owner should unregister the UX2.  Then you can register it to your own account.  If the original owner cannot unregister it for some reason, Line 6 can do it, but you may have to show them your purchase documentation, such as a receipt.

Licenses that are attached to the hardward, such as POD Farm will transfer with the UX2.

Note that any model pack keys that he purchased, and wants to transfer with the sale, should be transferred by the original owner to your account using the "gift" function.