EQTraders has a page entitled "
What to do with your Rares"
It doesn't offer a lot of advice on Yarrow. It is used to make Sackcloth but since it is a Tier 1 rare the amount of time even a spectacular piece of caster Very Light Armor is going to be worn doesn't seem to justify much expenditure. Look at the
pantaloons Some people will buy it for the novelty.
If you do use it then perhaps make the painting which - since it has no utility to start with can never get worse - will always be an unusual item.
Of course you need the rare and expensive Refining 4 book to use it.