The area you're in isn't particularly huge, so you may have a lot of competition in this field if there's only a couple computer shops, unless you simply want to help friends and family. If you aren't able to store the parts yourself, or through a third party, your profit margin is going to be slim because you're going to miss out on bulk discounts and take the difference or get contracts. You stated you aren't trying to make a living off this, though. This makes it a bit more applicable if you take specs down from friends and family, run up the prices online, tell them how much it will cost to order and build the PC, and charge a flat labor rate for the process regardless of how long it actually takes. This better enables you to control your funds since you're new to it.
Most of what I described to you is what I did for awhile, occasionally I would try to order parts in bulk that I knew would be used in modern builds for a discount then take the difference. It's quick, it's easy, and if you have a bigger interest in it and success with your customers, you can look into making it a more serious gig to include trouble-shooting and service.
