This page here is to give you the low down on all you need to know about this particular item. Mostly why it is valuable and why I felt like pointing it out to you. The only items that get their own pages like this are ones that you should definitely be looting and ones that require "other" explanation other than just vendor it.
For each of these items there are two methods of selling them, the quick and cheap way which is through barter. Type /bar or if you mouse over an item in your inventory the hotkey for bringing up the barter window on an item is Alt + Shift + left click.
The other method is through the The Bazaar. To bring up the Bazaar it is Control + Shift + Left Click. Check the value of the item in both to determine which you'd like to sell it in. Keep in mind that in order to run a trader you will need a Gold Account. Also keep in mind that if you sell in The Bazaar you'll almost always net a MUCH larger price than to a barter.
What are Essences of Life used for?
Essences of Life are the TBM version of Dreadmotes/Briny Essences. The essences of life are much more common than the TBM Briny Essences so it's strongly recommended that if you have the TBM expansion and can use these Rank 2 spell runes you purchase them over the Briny Essence counterparts.