As i see it Matt, you have sold a hell of a lot of Cosworth parts considering you dont have one and dont like them, and in your latest for sale post you personally state:

So you HAVE bought the parts specifically to sell them, therefore you are trading.
UK tax laws would also cite you as trading for profit and you would be required to declare all profits for taxation purposes.

Sorry mate, your a valued customer of mine and i hope you can keep these two jobs seperate, but I honestly cant see why you are even arguing here. Its not like you just bought a water pump or set of injectors for a quick buck, there are ELEVEN THOUSAND POUNDS WORTH of parts in your last advert alone man!