For example, this is a material required to make the orb you need to enter an expedition. I can sell/buy it in the Auction Hall. It's not BoP.
Skinning itself gives you not as much XP, but leveling up Tracking & Skinning gives you a lot more. Check out these screenshots...
Killing: 118xp
Skinning: 22xp
Leveling up Tracking & Skinning: 466xp
606 xp in 2 minutes.
I think it was for the lvl 35 expedition - some item "Stone ?? Orb", to craft it at the stonecutter a number of the shards that drop when you close a portal are needed, and something called "? Heart", which must be crafted, too.
Sorry, I didn't see the English terms, only the German (where a lot of items aren't translated yet, but have names like masterpiece_40_armor) and can't look it up, since it doesn't run on my computer.
The Orb and fragments are BoP, but not all materials that you need to make it are.