Saltpetre

The Saltpetre is a members-only Old School RuneScape item — “A power that can be added to compost to make fertiliser.”. Its Grand Exchange buy limit is 13,000 per 4 hours. Right now it buys at 5 gp and sells at 5 gp — a net margin of 0 gp (0.0% ROI) after the 2% Grand Exchange tax. High alchemy yields 3 gp.

To flip it, place a buy offer near 5 gp and a sell offer near 5 gp; the spread minus GE tax is your profit. Confidence figures are model estimates from live OSRS Wiki prices, not guarantees — flip responsibly.