It can be done, i have been doing it for almost 10 years as a hobby, mostly meadium to high end gameing systems. I dont make a fortune, but do make some extra ching every year, and constantly upgrade my own system with profits. Just make sure you can stand behind your build, explain your hardware and make very sure you know what your doing. Plus that the buyer understands you are not dell-lol, you dont have a tech line for them to call-and list and give them all warrenty,s on hardware, other wise i install mainly only the os, and limited free software to get them started. anything else is up to them if they want custom hardware or software installed than i will.
Also a big family helps with lots of kids that want to game to get started-lol. Than its college freinds, lazy gameing freinds, so on-lol. Ya it can work.
P.S. i have 4 systems in my house at all times, not includeing the 2 laptops. KIDS!!!!!!!!!!