The place to start is to make a good fountain pen (Cambridge, Jr. Gent, Triton, etc) and use it yourself. Try it with the kit nib, with a replacement steel nib, and with a 18K nib; try nibs from fine to broad to calligraphy and stub. Read everything you can about pens and nibs so you can comfortably throw around phrases like round nib, stub nib, italic nib, cursive italic nib, and oblique nib. Learn how to tell the difference between a right foot oblique italic and a left foot oblique italic.
Take some time to learn what goes wrong with pens and learn how to fix them. You might even want to try a little pen tuning. Make sure you know the difference between a wet writer and dry writer, learn how to fix a hard starter. Use your pen with an ink cartridge and an ink pump, try out 3-4 different types of ink. Take an ink supply assembly apart and put it back together a few times. Research a little about fountain pen history and learn the names of the classic brands.
When you get all of that done you can "talk fountain pen," and that is the starting point to be able to make and sell fountain pens. My experience is that pen users want to know that they are buying from a true fountain pen expert and aficionado. When you reach that point you will know what to make and how to sell them.