You could even get the opposite notion that you're just supposed to sell everything like junk because there's one or two NPCs that mention it's an important source and gil, and sort of give the impression you should be selling stuff to make money. One of the guys near Migelo's Sundries or something I think haha I forget, something about 'sell that junk if you want money.'
Which, mind you, quite a lot of it you can. It's still the main source of money and this is a game where money can often be hard to naturally come by... So, if you see anything over quantity 10 you should probably be selling it. But if you notice something you only get 1 or 3 of, especially after a few areas, could be good to hang onto it.