In my case, I did not take my car to a mechanic. The car I got was 1 owner off lease, 3 years old since first owner took delivery. The place I bought it from offers a free carfax report, a 3 month 4500 mile warranty(car was just out of manufacturer warranty), and they also have it inspected by another garage prior to handing it over to you. Granted none of this is "great". But I think it was enough to ensure that I was not getting a total junker. These things combined with a thorough inspection inside and out, and a thorough test drive, I felt confident in the purchase. So far things have been going good, so I will keep my fingers crossed.
I guess my point is, you don't need to be a "car guy" to buy a used car. You just need to have a little common sense, and work with someone you feel comfortable dealing with and trust. If at any point you feel like you are being duped, tricked, misled, etc, you should take your business elswhere. There are plenty of cars out there.