Is your car a standard or automatic? If it's a standard, check your neutral safety switch and your clutch safety switch. Make sure the switches were reconnected properly by the transmission shop. If your tranny is automatic, have the transmission shop look at the connections to make sure they...
I wouldn't ever buy a car without running a carfax on it first. Of course, if you like the car, take it to a certified mechanic and have it checked out. If the mechanic likes it, ask him how much HE would pay for the car. Then make your offer based on the mechanic's input. Never buy a car...