I would'nt buy a used car without seeing it first, and although I'm sure the "guy" is legitimate, there is a chance he is not.
To do things sensibly you either have to spend a few days state side and arrange everything, or organise someone/a business you trust to view/inspect the car for you, or alternatively, view the car yourself, arrange all the paperwork, but have someone else (or a shipping company) bring the car back for you.
If you end up incurring extra costs 'cos of any of this I hope the savings still make it all worthwhile - I'm sure they do.
Is there a chance you could view & pay for the car on a Friday, fax the paperwork the same day, enjoy a weekend in the States, and go through the border on Monday ?... or do USA customs need 3 working days to clear the paperwork !
M.