@IronicWeeabo
That's why I wrote
"buying either potions or raw materials for potions and duplicating them"
Just because he can't sell the duplication potion doesn't mean he can't use it for profit but they completely disregard that.
It would take a lot of effort to crash the value of consumables like a healing potion needed both in wars, adventuring and every day life.
Since the duplication potion is really cheap to create, he just has to buy 1 high tier potion or gemstone, turn them into 5 and then resell those.
Since his goal isn't to get super rich, he can easily do this without affecting the economy but still make tons of profit. Just keep doing it with different materials/potions and don't repeat it with the same unless a month has passed. But that'd change the whole story, maybe the mangaka shouldn't have introduced the concept of the duplication potion to begin with if he didn't want Red to use it.
So in short:
Maybe try reading it more thoroughly instead