I have bought about 7 or 8 engines through ebay, two of them (Saitos) had to be repaired at my expense, even that the seller claimed that they were in top conditions.
The first one, a Saito 72, needed a new carb, a ring and bearings, after repairs the cost of the engine and parts could almost be the same as a new one (I just do not want to know exactly how much I spent as I was having a great time learning how to repair it).
The second one, a saito 1.00 came like new, never used but the crankshaft were bent (a kind of 70 dlls for the crankshaft and maybe I will have to replace the bearings so this can go up to a 100 bill.
All other engines performed flawlessly.