You've all seen the large websites (no names needed) that sell assorted wheelsets at ridiculous prices. Some websites even do custom/semi-custom.

I wanted to build my own wheels, but it seems that I'd struggle to even buy the parts for less than I can get the entire wheel already built (and factoring in shipping too).

So, what is the deal?
- they buy in bulk? OK, easy to figure out.
- are the wheels all machine laced? I'd be getting them checked out anyway.
- some have slightly limited choices. For eg a rim/hub combo I want is only being offered with 32 front/32 rear spokes when I might have gone for less spokes at front
- anything else?

Is the price too good to beat, or should I pay more to do it exactly as I want?