yeah people love their over priced computers. Oh well, let them waste their money if they are too stupid to listen
Well, I think thats going a little far. Most people (quite rightly) would rather pay an extra $200 to buy from a reputable site who know their components, have tech support, and won't rip you off. Compare that with a random dude on ebay who says 'yeah I can build you a computer, just gimme some cash'. People who don't have the time or inclination to do the research/building/troubleshooting required for building their own computer are undoubtedly going to veer towards the safe, tried and tested method. And rightly so.
Thats the major obstacle in building computers for people, and I would say that its a bit of a pipe dream, really. If I could make £100 just for building someone a computer, of course I (or anyone here) would happily do it, in the same way that I'd happily get paid £100 to sit around an drink beer all day. Its just not really a particularly feasible way of making cash.
If you are really dead set on the idea, I guess the best way is to build it up locally, by building computers for people you know, and who will trust you with the investment. Then more work could come through word of mouth, etc.etc.