Food is traded very often on GE. For example, Lobster and Swordfish have a profit of 1-10 gp each. If you buy 15,000 every 4 hours, you can make more than 100k gp easily. The best time to buy them is at midnight before you sleep, when you sell them next day you wake up, you would make a profit.
Maple and willow logs have a profit of 1 gp each, and they are bought and sold very fast. Yew logs are 10 gp profit each, so the profit will be more than 100k for 12,000 logs. Oak logs usually have a profit of 1-10 gp each, so that would be 100k+ gp.
You can buy 13,000 coal every 4 hours, and it has a profit of 2-4 gp each. The current margin of runite ore is 53 gp, but because of the bots, you might not see the profit until early next morning or the day after.
Due to the constantly changing prices on the GE, it is strongly recommended to check the live prices on the GE before you do with this method.
Runes are great items to flip for F2P. For example, cosmic rune, chaos rune, death runes, and nature rune have a profit of 4 gp each, and they are bought and sold very fast on GE, so you can profit 40k from one rune in just 5 minutes.
