supplies a small number of expensive construction materials. One of these, gold leaves
is consistently profitable to buy from him and sell on the Grand Exchange
The easiest way to get to Keldagrim is with the Keldagrim trapdoor in the north-west corner of the Grand Exchange. After taking the cart, head to Keldagrim West and the stonemason's shop, which is in the southernmost part. Buy all of his limestone bricks -- this should take 18 trips to the bank, which very close by to the north. In the past his other stock has occasionally been profitable, but this is often not the case as those items see little use and can be obtained in small amounts at lower costs - either from the Bandit Duty Free shop in the case of marble blocks and gold leaf, or as a rare reward from Menaphite gift offerings for magic stone. Currently, marble blocks are not profitable, gold leaf is profitable, and magic stone is not profitable.
Once you've collected all the materials you want to buy, you can sell them on the Grand Exchange.