When evaluating fountain pens, the nib material is the most critical factor; 14k or 18k gold nibs offer superior flexibility and corrosion resistance, while stainless steel or iridium-tipped nibs provide durability for entry-level models. You must also verify the ink filling system, choosing between international standard cartridges, piston fillers, or vacuum converters based on your target market's preference. Additionally, ensure the feed mechanism is made of high-quality ebonite or ABS plastic to guarantee consistent ink flow without leaking or skipping.