It's hard nowadays. Everybody knows someones kid who is good with fixing PC's and can do it cheaper than you and there is PC repair places everywhere, that half rely on word on mouth, half on advertising and luck that someone finds you.
Cell phone/tablet/laptop repair is 10x the business PC repair is. Half the time people just throw out a PC and buy a new one. I surf the recycle area's sometimes and it's amazing how many towers you find in there that just had a dead HD or bad Windows on it.
Factor in it's hard to find other MB's and RAM for anything a few years old and people don't want to pay to repair it. Also, when dealing with Dell, HP's, etc, MB and PSU replacements can be hard or impossible to replace and thus require a new PC as well.
Then say you can't repair it, and talk them into a building a new PC. Do you order parts from a pcpartsbuilder list, then you have to wait a week for the parts to come in and pay for them out of pocket. What if no one comes back and buys the PC? Do you ask for a deposit? Unless you start doing credit card and debit card's, then have to go take out cash, give you a big deposit to cover the parts cost, wait a week, and then get their PC. 75% of people will say screw that and go to staples/best buy/dell and buy a PC and have it that day without having to run around to debit machines and take out cash and wait a week. I know, I have a hard time trying to talk people into a new PC nowadays in my shop. It's also hard to compete price wise. When you have to add a legit Windows to a system cost, that add's a lot for the small builder.
Good luck, start off small and see what you can do and if you can get any business or people calling. Don't go rent a space, buy 10k in inventory like HD's and RAM to have it in stock, and you may have a small chance.
It's hard. I have expanded to fixing XBox's, PS3's, cell phones, laptops, various electronics, PC's, e-bikes, whatever I can fix for a buck
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