DO NOT BUY USED ENGINES!!!!!!!!!!!!!
You are gambling big time. you dont know what abuse that used engine has been through. I can tell you now, it looks like a nic on the piston on the right side of the pic. That there is either broken in wrong (which you dont want) or its more than broken in...
I have 3 engines at home that I have broke in - and they are still shinny at the top of the piston.