Service tax or vat registration for second hand car dealer

A dealer deals in Second hand car trading. he purchases second hand car from different individual (no one gives bills hence NO VAT paid by him). Now he sell that cars after furnishing. Now he wish to register under VAT , so at the time selling he will issue invoice on which VAT shall be charged. My question is whether we should register him under VAT (MONTHLY TURNOVER APPROX 50 LACS ---VAT 5 % ) 'OR' WE SHOULD SHOW HIS INCOME AS 'COMMISSION INCOME' --> MONTHLY APPROX 7-8 LACS [THEREBY REGISTER HIM UNDER SERVICE TAX @ 15 % ] ??