I just bought a used bike off Craigs List. Everything seemed ok. I took it for a test drive and all the gears shifted fine and the brakes worked and there were no cracks in the frame. Upon getting it home I noticed that the bike would not roll backwords. The rear wheel will not spin backwords at all and if I try it looks like the chain is tensing up and the shifter starts to push out like it is under stress.

If I petal backwords the chain loses slack. What is wrong, PLEASE tell me this is an easy fix and that I didnt just get ripped off.