A few ways to earn money quick. On Grand Exchange

[Khala]

These are way to make money fast, they are all close to the grand exchange.

Chocolate bars.
They are located in the cooking guide.
They sell for 100gp +
You can work on your cooking exp while waiting for it to respawn.

Logs, yes logs.
1 Log sells for about 30 gp.
1 full inventory is worth about .2k
So 100 logs is about 3k

Clay
Clay is easy to mine, and is high on demand.
Each one sells for about 95 gp.
You might even get a gem from time to time.
So you can make about 9k from 100.

Oak in now even in a higher demand than logs.
Almost every tree gives you more than 1.
They sell for 10-15 gp each.

Silver
Silver ore sells fast and are worth about 150 each in the exchange.

Some Others:
Planks.
They spawn in the wilderness.
Some locations are:
Ruins just outside clan wars.
And Graveyard next to Bounty Hunters.
There is a bank located at bounty hunters.
Watch out for revenants.

There are many other ways to make gp, like picking up red spider eggs in the Varraock Sewers, or selling Limpwurt Roots.
You can buy rune item or higher cheap, try selling them at normal price to make profit.

You can experiment with many other items to suit your playing style.

[Rimik-]

Also selling pure ess is a good way to go.

[♂Tomy♂]

nicely done but always buying santa at 11 mil then waiting for it to go to 20 good to lol