Re: What is the best bike to build to sell?
Well, you can't beat a good used bike I suppose.
My personal criterea when building a bike for sale would be one with good wheels and good brakes, which is why I like the Grubee GT1's and GT2's (not sure what the deal is at usbikes, btw). I own a GT2 and while some components seem to be "walmart" standard for quality, like the crank and head tube bearings, the wheels are very sturdy and much better quality than most "cheep" bikes you can purchase today. Currently my GT2 is stripped down receiving a new "Bomber" themed paint job. If I had to do it again I think I'd go for the GT2, though. The built in tank on the GT2 is nice, but I actually prefer the teardrop tanks that come with the kits.
If I couldn't build a bike to resell using a Grubee bike, I'd have to go with a "bike shop" quality bike just to be sure I had a good safe foundation bike. The Specialized Hardrock certainly qualifies and Craig's list is probably where I'd go to find one.
To be honest, I'm a little nervous about building bikes for others because if the liability issues. I have three for myself and one I built for my son. Soon I will probably be building another for a brother and one for a friend but I'm not sure I could do many beyond that.