Apart from the SORN issue I don't think DVLA are that fussed about whether someone is a "Trader" or not. The people with an eye on this are A) the tax man and B) the local Trading Standards Officers. Someone buying a car from a legitimate Trader enjoys additional rights that he doesn't have when buying from an individual.

The TSOs want to make sure Traders aren't ducking their responsibilities, and legitimate Traders are putting pressure on for them to do so in order to level the playing field - as Traders on here know, it's difficult to compete against someone with minimal overheads and no worries about disappearing off into the night!