As Metcars says, the selling is the easy bit. It's the buying where you make your money. You are competing with huge numbers of 'traders' all trying to find the same bits for stock. Legitimately or otherwise. The pitfalls are many:
Sellers lie
Cars can go wrong; anytime, anywhere. Never convenient.
Cashflow controls everything
Buyers lie
Overheads
General expenses
HMRC
FCA
'Experts'
Conmen, thieves, tax-dodgers, halfwits and lunatics. And not just the ones you employ.
Other than those, if you could get rid of the paperwork and the customers, it'd be a cracking job!