It used to be the rule that the vehicle had to be taxed, mot'd and insured for the road before you could have the registration no. put on retention. Since the introduction of Historic Vehicle status and no MOT required, it's opened up this kind of sharp practice.
I took my V5C for my 1955 Ariel Red Hunter to the post office to get the V5C updated to Historic Vehicle. The Postmaster zapped the bar code and before I knew it, it was updated, licenced and road legal. (It was of course on my Classic Insurance policy). He said, there you are. V5C updated, taxed etc. so you can continue riding it while they process and send you the updated V5C!
Only problem was, it was still in bits in my garage and loft so that may have been a bit difficult.
So, all that purchaser needed to do was put it on his classic insurance and DVLA would be happy to put on retention for him to sell the number.
By the way, the reg. no for my Ariel ASX 95 has been valued as being worth about half the value of the bike. As far as I'm concerned it will stay with the bike as long as I have it and I hope if I sell it on, the next owner will keep it too.
There are more important things in life than money!
Chris