how can I get a serial number to activate my hardware?
Here is a link to our registration support: https://bit.ly/NIsupport_RA
Try to contact NI support.
One time I had a similar problem with a Maschine Mikro and what they asked me is some things to prove them it was in my possession (the old serial number that was under the Maschine and a picture) and then they helped me with a new serial to register it
But if you are still in contact with the seller, the Audio 6 should be registered under his account. What he has to do is simply ask NI for a License transfer and they should give him a new serial to forward to you to allow your registration
https://support.native-instruments.com/hc/en-us/articles/209566129-How-to-Request-a-License-Transfer-ID-to-Sell-Your-NI-Product
