I think most of the advises on here about providing excellent customer service is 100% true. You're providing a service and it's your job to make sure that provided service is 100%. After reading the comments, I feel like this business is tough and very hard to make a great living. If you still want to do it I would advice you to instead of gunsmithing, you would want to focus on manufacturing parts. I think you have a better chance by designing your aftermarket parts (say for Glock, Sig), test it very well (this is the most crucial part, you have to have an engineer mindset and develop extremely proficient test cases), then have China makes them for you according to your specifications. You can then rent a cheap warehouse for storage and dump them on eBay/Amazon/YourOwnWebsite. Good luck!