There are other ways to make a decent living and don't have the computer headaches as noted above.
Let your clients find their computers themselves and you provide services afterwards. Those services are worth gold.
You don't want to build the systems, too many variables to get blamed for. Better to fix them after warranty is over.
This doesn't involve inventory, which is the biggest overhead you start with. A losing proposition.
Offer antivirus and spyware removals which are quaranteed business makers. Offer service contracts to renew AV and spyware protection.
Advertise free diagnostics to get fast business.
Run free diagnostics for malware and the like. Have clients drop off their computers at your place and go from there. Then call the owners and give them the bad news.
They will usually let you do the work or just leave their computers for you to sell later for your trouble. Great Ubuntu candidates.
Since prices for computers are dropping, there is a max price line that people won't go past before considering a new buy.
Offer to backup files services, clone harddrives, add slaves, copy to larger drives, and so on. These can be money makers and your clients will love you.
These services can be repeated over and over. What you charge depends on what the people will bear. Look for repeats and word of mouth.
Be interested in your clients problems and make sure you know what you're doing first, so I recommend a course in computer repair and diagnosis. If unavailable, then a stint at your local computer repair shop, first free for experience and go from there. If you got it already, good for you.
Good Luck and Happy computing.