why does it matter if you are are registed as a business seller or a private seller as long as you pay what you owe the tax and vat

for example i sold my van on ebay the other week
http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=280131163107
but it clearly states the final price is plus vat...so whats the problem

i also sell motorhomes on ebay but make it clear i am a dealer