if your selling parts to fund your race car then it will still be classed as trading, ie; your earning money from it to fund something else.... a good few years back i got quizzed about trading because i had so much stuff for sale but had 6 odd RS's at the time so had a valid reason as most of the parts were bits that had been replaced on the cars. I now have to pay the trader fee to have the company name in my sig.
i did think maybe PF could have a platinum membership for thoe who want to sell a bit more but this would encorage bedroom traders.