Call the seller. Don't just email, start evaluating the seller. Things to ask: http://www.carbuyingtips.com/questions.html (though that list is horribly extensive; I'd rather just pick the three best-looking candidates and do that thing on the spot; you'll need to doublecheck anyway to make sure the seller wasn't lying).

Check the CARFAX report for the car. Go for the $24.95 30-day option, you should always look at more than one car.

Check the smog check history (fails are a sign of trouble). In California, in most of the cases, the seller is required to provide a certificate of check newer than 90 days old -- it seems the information flow to website is slow enough that this latest check does not show up, or something.

to be really careful, you should check a lot of things like

bring a car nut friend who knows what to check; play good cop bad cop (this checklist is not as good, as I am not a car nut)

you have to drive the car; leave a photocopy of your drivers license if needed for assurance

for some things to check, read more , especially On-the-Lot Checklist , Road and Test Checklist

Make an appointment and go see the car. In good sunlight, you want to see the car.

Take the car to a mechanic of your choosing for evaluation. Alternatively, choose to trust on service from brand name vendor.

If you know what you're willing to pay, go to your bank and get a cashiers check. Or two alternatives, if that works out. Otherwise, you will need to go back to the bank after haggling, and without a deposit the seller may have sold the car to someone else. Or something. Tuff.

(Some people say haggling at dealerships is easier if you show up with a cashiers check just a bit short from what they're asking.)

Don't pay with cash, there's even less chance of getting it back than cancelling a check.

If you happen to be a seller reading this, do the actual sale in a bank to be sure you aren't being cheated.