Eh.
I don't think Buenichico's answer answers the question at all and it certainly doesn't say what you seem to think it says.
Since second hand car parts do not fall into the category of goods on which a margin scheme may be applied then in fact they would fall to have VAT charged on them in full so far as I am aware. Even if they DID fall into a margin scheme there would still be VAT payable on the profit on them.
Second hand parts should be treated exactly the same as brand new parts. VAT is payable in full on them when they are bought and sold (unless the trader in question isn't vat registered of course).