There aren't many people interested in getting a custom computer built. Most aren't aware it is an option. Most of those interested are younger and have a friend who will do it for a couple beers.
Doing computer repair is a better option, if you are qualified. Provide the option of custom building if it is time for a new computer. I'd recommend having them buy the parts and you assemble it plus install the OS/drivers for a set fee. The last thing you want to do is provide a warranty yourself. Then you'll have some nitwit calling for free repair service. When the issue is software (not covered).
The best place to post is physical mailers but word of mouth and repeats are the real sellers. Online advertising is useless. Craigslist will...