Personally I wouldn't.
There are a lot of people on ebay who will try to rip you off. Most reputable engine dealers will provide a start up warranty, make sure all of the censors are in working order, and the engine will (in general) have a lot less mileage.
There's no real way to sue someone from ebay either if you get a defective motor, because they can just say that "we didn't say it was all in working order" or some BS like that... plus you're not going to get tech-support like most dealers will give you also.
You will probably get the motor for less, but you get what you pay for.
I would look into private sectors (like these forums perhaps) depending on what you are looking for... and there are a few good websites that sell engines and have positive feedback from thier customers.
Check the reference or swap sections of this site, there's a list of dealers somewhere.