I've forgotten the exact prices at this point (I want to say buying mid-20's per and selling mid-30's but that may be complete misremembering). The basic idea was hanging out in the dwarf mines where people are mining coal and giving low offers on coal, with a promise to increase price if they can sell in quantity (and in certs). Then, just turn around and sell the certs, once you had a lot of them, at a higher price in Varrock town square (or similar).

One other reason we thought this wouldn't work is that it seemed to us buying in large quantity should get a discount - bulk discount, right? But that wasn't the case. It was cheaper, per unit, to buy hundreds of coal than tens of thousands.