1. home built PC market is probably larger than the entire Mac market. I don't have any figures to back this up but based on Windows' market share and my experience, 20% of PC's I've seen over the years are home built.
2. Those "home built" PCs are also built and sold by PC shops as complete computers, not necessarily home builders. My father ran a business that built OEM white box PCs for 15 years. He sold over 200,000 units.
There is going to be some overlap. Particularly as most of the killer apps are entirely cross platform.