I have the same endeavor... There is a kart shop 5 min away, another 20 min away, and several an hour away with jc specialty still dominating... So my focus is body work lettering and tires rather than stock everything and anything
Unless you buy in bulk and keep a good inventory you will struggle to compete with the big shops, BMI, JC, Turk, ect. We have sold parts for years, do it mostly as a hobby and helps offset our cost of racing. Do we make a living at it HECK NO!