Where can you sell Rune Essence in RuneScape?

Rune essence can be mined through the Mining skill on non-members worlds, while pure essence is mined on members' worlds by players with at least level 30 Mining.
How to get

Rune essence is only minable when you have completed the Rune Mysteries quest. Talk to Aubury in Varrock or Sedridor in the basement of the Wizard's Tower. They will teleport you to the rune essence mine.

On the other hand, for runecrafters level 50 and above, you can buy essences for the Runecrafting Guild with Runecrafting guild tokens from playing The Great Orb Project. 1 token for 1 essence, or Pure essence(members only).

There is also rune essence in the dungeon on Moonclan Island.

Rune essence is a fairly rare drop from minotaurs and catablepons in the Stronghold of Security. Ankous also drop it, usually more often than the rest of the Stronghold's monsters.