How longs a piece of string?
This kinda question can't really be answered.
Plenty of people have purchased well built engines and they run fine, others have purchased cheap built engines and they haven't run fine.
Depends who built it, how it was built, how long ago it was built, what parts was it built with, what power it was built for etc etc etc.
Always get it tested before buying it and ask for receipts. But in the end there is always risk with second hand goods