Hey there Biker...welcome to motorbicycling.
There are a lot of things to keep in mind when you make your purchase...least of all should be the bottom dollar on eBay, unless you don't mind dealing with headaches and issues.
Your best bet is to ask questions (lots), read as much information as you can with regards to what is available out there. If you can find any reviews, read them carefully and compare what you find out to other engines and dealers. The other good thing is to ask other riders what engines they are riding, and if they have had any issues with the engine or the dealer. Consider the warranty that is being offered, and what it consists of...does it seem fair?
Also, ask yourself about what you are purchasing. Does it make sense that you could really purchase an engine of decent quality for that price?...chances are if the price seems almost to good to be true, it likely is.
Cruiser is more than right when he says that not all these engines are the same...even though they may look very similar...it's what happens on the inside of that engine that decides if it is quality or not.
I hope the above information (or lack of) didn't sound like any type of sales pitch. The link for my website is in my signature for that purpose, lol. I am just offering advice on how to find a good engine.
Naturally, I would say that my engines are the best there is, but that wouldn't be a good thing to say unless my customers say it too...and they do.
If you have any questions, please feel free to get in touch with me.