Same potion. Same doses. Different bottles. That is where the price gap hides. A four-dose potion does not always cost four times as much as a one-dose potion. You can trade that difference. No monster to kill. No potion to brew. Just the right bottles, a decanter, and a gap that survives tax.
OSRS-GE is a free Old School RuneScape Grand Exchange tracker. Prices come from the OSRS Wiki Real-time Prices API and refresh every minute. Every margin shown subtracts the 2% GE tax, capped at 5m gp per item, and respects each item's 4-hour buy limit, so you see the profit you can actually make. Use the calculators to compare flipping, overnight offers, High Alchemy, herb cleaning, decanting, Plank Make, tanning and other money making methods, and read the guides to learn margin checking, how to avoid fake margins and when to trade.