I have purchased stuff on e-bay and I have (knock on wood) never been burned. But consider this before you buy. How much is shipping? What sort of warranty do you have? how is this warranty resolved? Will you have to box it up and send it back or have it fixed localy and send them the reciept?
Best bet on an item like this is to locate a used short engine and then have it rebuilt by a local machine shop. Then you know for sure what you have and you have someone you can get face to face with in the event there is a problem.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"