I think upload a picture so we can see the issue... Others are right though, if he gave a deposit a week before then turned up and you had got the V5 sorted, I would be a bit cheesed off.
The vin plate issue could well be him just trying to knock a few quid off. He is a trader and traders are there to make money.
Threats are not cool though. No one should be made to feel threatened and its down to the person but some people dont feel comfortable with it. I love them...

But thats a different matter.
I think, try to sort the issue out with him directly and Ata, when your back on, try to do the same mate. At the end of the day, your not a bad seller on here, regardless of what has gone on in the past. I have had bits off you just like loads of others have too and everyone has been happy from what I can see.
No point in getting banned every so often for not keeping your cool, especially when the zone can create a good income for you seeing as its the largest place for E30 owners...