It's basically near impossible to do for three reasons.
The first is that if someone's going to buy a pre-assembled computer and can't build their own, they won't understand why yours is better than a Dell, so they'll buy a Dell. People who know enough about computers to understand also won't be interested; they'd rather just build their own machine for cheaper.
The second is because most people who would buy a Dell can't troubleshoot their own machines and aren't interested in learning how to, so they'll buy from Dell or at worst a local computer shop so they have someone else around to do support for them.
The third is that, as mentioned above, actual computer shops get wholesale prices on their parts. As an individual, you can still beat their prices with what you get retail off Newegg, but the margin is very thin.
Of course, you can sign up for a commercial license and you can provide support for your buyers, but by then you're basically running a computer shop already.