Saltpetre

Saltpetre last sold for 2 gp (instant sell) and was last bought for 4 gp (instant buy) on the Grand Exchange.

You can buy 13,000 Saltpetre every 4 hours. The 4 hours start from your first purchase, and only buying counts towards the limit.

High alchemy turns Saltpetre into 3 gp. Buying one at 4 gp and casting with a nature rune at 138 gp costs 142 gp. That is a loss of 139 gp per cast at the latest prices, so it is not worth alching for profit right now.

It is not on the exempt list, but it pays no tax at its price. Selling one at 4 gp costs no tax. Items selling under 50 gp pay none, because 2% of the price rounds down to 0 gp.

The highest daily guide price on record for Saltpetre is 429 gp, on 9 Jun 2016. The lowest is 2 gp, on 4 Jul 2018.