Buying a used engine is kinda like the Clint Eastwood line " Feeling lucky, punk? Well, are ya?".
Yes, you can get a good engine at a great price. OR, you can get an engine that looks brand new but is a paperweight. It's so easy to riun a nitro engine. A siece of debris down the carb on the first tank is al it takes. On top of that is the inherent risk that goes with any e-bay purchase.
If you're serious about that engine then I suggest that you do some research to find out what parts would cost for a full rebuild and factor that into what your bid will be.
Bottom line, buying anything off e-bay is a gamble and buying a used nitro engine is a gamble. So, if you're a gambling man it's probably worth the risk....But it is a risk, none the less.