Yeah, it's worth like 10 Gil right? Apprently the Great Serpentskin is supposed to be exchanged to the Nurse Lady or Traveler and he gives you the Barheim Passage Key which leads to the Zalera Esper. Funny thing is, I've almost beat the game 100%, gotten the Barheim Key, and I still have the Great Serpent Skin in my Loot list. I'm hanging onto it.