3 problems:
1. There might be legal issues with it. OS X's EULA is pretty tightly written, you can't run it on machines that aren't Apple.
2. These are spare Mac parts. They don't have the warrenty of the real things, you'll probably have a harder time finding a shop that will service them(try getting an Apple Store to do it), they probably won't be newer high-end parts, and Apple can very easily cut off his supply.
3. Apple is a hardware company. Clones being sucessful might very well kill OS X.