DO NOT purchase a car unless it is through ebay. And DO NOT send cash to anyone through any service even if the ebay emails they send you look completely valid. Go through ebay (and you contact them, do not let the seller send you an ebay link or an email that looks valid) and use paypal so you are protected. If the seller is not willing to do that then it is a scam.
The seller should give you a number that you then take to ebay yourself and log in. You can forward ANY email to ebay you are uncertain of and they will respond very very quickly to let you know if it is real or not.
I know lots of folks who have purchased a vehicle online and been incredibly pleased, and I know folks who have lost every penny. If it seems to good to be true, it is. If they insist you pay cash through a wiring service, it is a scam.
Good luck
V. S