I think that if you want to do that, what you need to do is really to know computer repair, etc. When something breaks down for someone, then you can push the idea that look, these prebuilts are made to last 13 months to get people out of warranty, and then when they die, people are stuck.
So you push the quality issue for them, saying hey, it will cost a little more, but if I build one, I will be using higher quality parts, plus you get local service. A lot of people like having a local guy do things still because local shops can afford to take more time with them.