The best way to earn money in runescape is through merchanting. The following are several ways that I have found wok very well.
The first way is through law runes. Laws are fairly common to members, and if they finish the quest, "The Feud" they have an even better way to get them. But non-members can't buy them anywhere and can only find them from monster drops, so they are quite rare. Anyway, you get several hundred laws, or as many as you are able to buy on a member world, and then log out and switch to a non-member world. The runes that you have just bought for 300gp in the member world are now worth 500gp - 1k. I advise to sell them in quantities of 50 or 100 because if the quantities are larger, no one will buy them.
The second merchanting tactic is to buy and sell coal. Coal can sell anywhere from 100gp to 300gp. So, naturally, you buy the coal for 100-150gp, and sell it for 300. I advise mining your own coal just to be safe, but buying it does work.
The third good way to merchant is to buy and sell god armor or dragon weapons, anything that has a price value of at least 100k or more. D-longs work well because many people don't do the lost city quest, and can't get them easily. you can buy the d-longs from either players for 90-100k or get them from the lost city for 100k always. Once you have them, you can sell for anywhere between 105k-130k making you a sure profit.
The third way is time consuming, and not really merchanting, but it is worth it. Flax is an item that members are commonly in need of. you can pick the flax just south of Camelot and deposit it in the bank in Camelot. Flax sells for 100gp each, and requires no skills to retrieve. If you pick 28 in one inventory, that is 2.8k. You pick a lot of flax, 1-10k , and sell it for 100k-1mil.
