So, a player (Zweihander RPG, fantasy, very low-power, very low-magic)
How low magic are we talking? Any casters in the group at all? Screwing with the market on spell components would be a good idea in most settings. There's lots of ways to play games with it. Buying all the guano to stop casters from using fireball is easy, and just makes the party travel farther for it or pay more money. But what if about 10% of the guano on the market was counterfeit and caused the spell to fail? If guano is rare, what if someone gave the party a false lead on where to travel to find it? What if someone pays local ruffians to stop picking pockets for wallets and had them start picking pockets for guano instead? If she's a noble, what if she starts a political campaign to ban trade in guano (for animal rights reasons, or something similar), or gets it heavily taxed? I'm using D&D components for simplicity here, obviously. Substitute as needed.