I used to manage a PC building workshop, and even with our buying power of £30,000 worth of hardware orders a fortnight, we still were not buying enough to give us a great profit on hardware.
To beat the big guys you would need to be ordering literally thousands of components at the one time, which would cost you hundreds of thousands of pounds.
The money was mostly made with the warranty. But even that was a headache as people with a computer warranty will be on the phone 24/7 moaning about the computer running slow, or a pop-up thing, or a slight buzzing noise, blah blah.
The whole ordeal was a sore head and the pc supply side of the business barely made a penny. The company only survived because it got paid to train individuals in using the PC which was golden.
Not to discourage you, or anything!
Maybe you could try and add a unique approach to your idea. Making custom built PC's with a visual design aspect. Then your charging people for the same old hardware but an artistic theme. More or less what Apple do