Thank you. It is true that the seller did not reply to me initially, but did end up confirming a few days later that he unregistered the device (weeks before he sold it to me, but double-checked to appease me), and as of right now, I still cannot register the device from my end. I do see that on Arturia's site, it states that if a re-sell happens within 6 months of original purchase, we will have to go through their support to get a device registered, so maybe that is the problem. But given that I have the actual keyboard and all of the original included documentation that comes with the keyboard, and sent them photos of all documentation / serials / unlock codes, this really shouldn't be so challenging. I'll maintain hope that I hear from someone eventually, but the lack of any human response after a week has been really off-putting thus far.
I know that Arturia isn't going to get involved in private sale issues, but they do advertise a streamlined process for transferring device ownership, and so far, they have really failed to deliver on that.