This is a method from http://www.thersmg.com/
. Since you guys here at Sythe are a more intelligent crowd, I'm gonna summarize it.
You are basically gonna kill Warped Bats and sell their Batwing for money. Batwing is used to make the new Batwing Armor for F2P (currently the best mage gear in F2P), so it's in hot demand.
1.) Gear up in your highest damaging gear and a ring of dueling. Go to the dungeon just south of lumbridge palace (pictured below).
2.) Kill those Warped Bats. Pick up just their Batwing (100% drop).
3.) Use ring of dueling to bank. Use home teleport to come back to dungeon entrance and repeat.
Even though the GE price is ~1500, the street price is 3000gp each batwing:
For full method details and profit calculations, original guide can be found here
. Have fun!