I never buy.
I'm a collector IRL(basically a severe form of "but I might need it later") which automatically carries over to how I play games. Fortunately it's not THAT bad for me(and in the last years it's improved a little bit) but to keep this about games, I usually finish Final Fantasy games with EVERY consumable I've collected and not a single one used(well, in an area where I could easily get a potion, I might use one and farm 2 extra, 1 when at 99). I would not buy a shortsword to replace my stick, simply because there might be a longsword drop.(but when I get the longsword, the stick will go) I usually finish games with millions of gold simply because the one time I might buy an item(costs maybe 50 gold when I have 100k) I will farm twice the amount of money spent to compensate.
The only exception is an inn for the simple reason that a full heal costs less than the potions I would have had to spend on it. Games with a duplication glitch are also an exception in that I will carry a stack with the item limit and have at least six stacks of that item in the storage I will then always "cheat" the items I've spent back.