I did personal, retail, and SMB PC repair for years. It paid quite well, providing I did it for myself, and not someone else; going rate for "GeekSquad" is probably $10/hr, if you're lucky.
Make sure you file for an EIN, are fully bonded, and pay your state taxes.
Buy about $1,000 of semi-decent parts from an 'overstock-and-end-of-line' reseller like
CompGeeks. An $8 modem that you sell for $15 is still $10 cheaper than buying it at Best Buy, and they'll realize that you're not fucking them, even if it is nearly a 100% markup (you must consider the storage, shipping, and taxes you paid) - it's still cheaper than retail.
It can't hurt to put out a few ads in the local free newspapers, such as "Big Nickel", "Thrifty Nickel", etc. However, most of your work will come from word of mouth if you keep your clients happy.
Sometimes you might get called out to fix a problem that's as simple as a cable being disconnected. If you don't charge for those, or pro-rate good customers, you'll have them for life. You will, however, have many idiots try to defraud you, and waste your time. Ensure that they know that once you leave, unless a piece of hardware 'dies', it will be another service call.
I haven't offered these services for years, and I still get email from former clients seeing how life as progressed, and often, begging me to come back and fix things.