How do you go about telling the taxman your trading? Do you need to be a registered business?

I'm not sure it's worth all the hassle. Especially if I need to offer a warranty. I've never tucked anyone up on a car so it shouldn't be an issue but if some idiot buy's something Turbo'd off me, up's the boost and kills the engine, would I be liable? I sold an M3 to a joker once that told me he was going to do 150mph in it on his way home. 10 minutes later he mentions if the engine lets go on the way back, he's going to bring it back to me, nob.

It's as much as a hobby as anything. In the past i've only been buying cars I actually want to own, buying them needing work, fixing them, run them for a bit and sell them on for a profit (although depending on how long I decide to run them, don't always make a profit).

I'm not so worried about any technicality's with regards to the insurance, it's the government i'm more bothered about.