it would depend on if u yourself new its history (like if it was your mates) and how they ran it (u dont want a thrashed engine) and have they had any experince with a petrol car before and knew what they where doing with it, if all that checks out then i would start tinking bout it and asking to have a look at its inside and look for any sings of wear but if it has been missused u could say look mate its worn (and tell him i will need to spend money on it after i get it from u) and get it a bit cheaper and then put a new piston and sleve in it or bearings or whatever. because one fella fron radio controlzone bught a engine of ebay and the add said it has only had a half a tank of juice though it and it was kept reallly well and when he got it the first thing he did was strip it and the berings where shot the crank case was stuffed and there was a giant dimpel in the piston (due to a shaft loking tool that hold the sharft so u can take the flywheel nut off) and he hasent got very far oout of this situwation i would be real dussy bout it if u r going to buy second hand parts u either want it of a mate or from a good shop that u can take it back to or a non moving part (if u know what imean like a carbon fiber top deck or something like that something that aint going to wear out
hope this helps