jp
I have bought a few second hand engines in the past with no worry at all. in my opinion the jp motor is an awesome motor and i have one myself. When i buy a second hand motor, with the owners permission i pull the back plate off and inpsect the rod. Spin the motor to top dead center then rotate the crank back and forward and observe the play in the big end. If it has more than a few degrees 10-12 it may need replacing. Also observe if there is any dirt and crap inside the motor. pull the head off pop the liner out followed by the piston and rod. look for any marks on the side of the piston. check for any cracks between transfer ports in the liner.. Pop the crank out and feel if the bearing are gritty or grindy. sometimes you can pick up bargains buying second hand motors if you have a bit of an idea on what to look for. Also before you pull it apart pull out the glow plug an spin the motor over and feel how much nip it has at tdc. My jp black has next to no nip at all yet when you put the glow plug in it has a lot on compression.
I have got into a habbit of pulling the engine right down and cleaning the block / crank in an ultrasonic cleaner after every race meat. it allows me to keep an eye on things on the inside and gets rids of minute dust particals and foreign material. I mainly do this on my jpfx ro3 drag engine cause it is so hard on rods. If you get in the habbit of maintaining stuff properly you will find this hobby becomes a fair bit cheaper and more enjoyable when problems become few and far between.
as you are new to the hobby it may be worth while getting the owner of the motor to pull it down in front of you. Most guys will happily do so if they have nothing to hide.
Just my 2 cents worth. anyone have anything to add????