I believe the letters can be turned in for some sort of reward when you have a certain number of them....I don't think anyone does this anymore.
The slippery boar oil you can sell on the AH or to a vendor for a small amount of gold. I believe it is a material used in some special event that occurs a couple weeks once a year, so you can save it or dispose of it.
The boar oil is for the Christmas event. not really worth keeping as you don't need a TON of it, and its super easy to farm.