your issue will be that you are too late to join the game!
There are hundreds of very reputable specialist PC builders out there now that build to exact requirements or budget level PC's, and also offer top notch customer servie and warrantys. The choice is no longer just the mainstream Pc world, comet etc.
You can still have fun and maybe make a slim bit of prifit but i doubt you would make much if anything now tbh. Do you really want the hassle of people coming back to you when some peice of hardware goes kaput and start arguing about who is responsible for the repairs?