Is it possible to sell one of the key fragments? I've been through the five prerequisite tombs, several times, and definitely missed nothing. I've even tried selling one of the four I have to see if it's possible to sell, but I can't find them in my inventory! What did I do? How do I find a fifth key that I've already found, but somehow misplaced?
Are you sure you went through the five prerequisite tombs? You might be counting the westernmost cave symbol as a tomb when it's not; it's a ruin and is not part of the quest.
Here are the locations of the five tombs housing Fairel's Tomb key fragments:
As I said, the westernmost cave symbol depicts a ruin and the easternmost is Fairel's Tomb proper.