If your magic level is 55 or above then you will have the ability to cast the magic
spell “high-level alchemy”. The spell requires 5 fire runes and 1 nature rune (wield a fire staff to eliminate the need for fire runes). With this spell, you can change an item into gps at the rate of 1.5 times its original value that you can sell it for in the General stores. Buy an item (the higher the quality the more money you’ll make) at its original price (the original price is the price that the General stores will offer for it if it doesn’t have any extra stocks for the item) and then cast the spell on it. It is as simple as that. As an example,say you buy a rune medium helmet for 8k. You can then use the spell on it and gather approximately 11k (rune meds can be sold in general stores for an estimated price of 7.5k) for the helmet.

Formula:
P = OP * 1.5 – PB

P means profit, OP means original price in store, PB stands for the price you bought for.