The AIR site has details on inMusic's general license transfer policies:
https://support.airmusictech.com/en/sup ... a-new-userSo, if he sold you the hardware he should have instigated a transfer to you directly through Akai, although my understanding is that if you provide a valid hardware serial in your account they normally just assume you are the new owner of a second hand unit. At that point you would have official ownership of the hardware and any software that
originally came with the hardware.
Beyond that, if he had later purchased some plugins and left them installed in the MPC, it seems (from reading the above) that you are not entitled to those particular plugin licenses unless the owner separately transferred them to you through Akai, you can't just acquire them because you bought the MPC as they were never part of the original hardware package. As far as Akai are concerned, you only bought an MPC. You need to get the previous owner to transfer the additional plugin licenses to you through Akai, as currently they will be tied to his inMusic account, not yours.