Id absolutely love this. But i can see the task being daunting as everyone is really picky about what they get for their builds. Im sure some parts would sell better than others. Obviously GPUs do well here already. The wait time, as mentioned, could be an issue here. For people doing a build and not upgrading, the wait time could potentially have them without a machine for a few weeks to a few months. Persoanlly, Id shell out any extra needed to have my PC up and running ASAP.
Upgrades are another story though. Things like GPUs, RAM, Fans and Water Coolers, would probably sell better, as they are things that are upgraded frequently. I can see cases and CPUs also selling well as upgrades if the deal was good enough. Also, the mechanical keyboard community here, and other tech items like mice and monitors would also fit perfectly into the PC Community already, so there is almost enough parts to start one as it is really.
I agree the community here is great. I think if Massdrop was able to get items made with community input, maybe a custom color case or cooler, etc, on parts that would benefit builders as well as upgraders, they could really make a splash in the very competitive market of PC parts.