I was once trying to make a Tournesol and one of the ingredients is 3 Gemsteels. As luck would have it, I had already acquired two, but I accidently used them up in making a duplicate of a katana that I already had, so I had to farm the ingredients for 3 more.

Let's say you get three Gemsteels and sell all of them to the Bazaar and make that katana. You'll have one left over, right? Wrong! As soon as you make an item, the Bazaar uses up all the ingredients, so that every Gemsteel in the Bazaar inventory is used up. So if you need just two to make an item, sell two and save the rest as loot.