I recall reading a thread somewhere on this forum the other day, dont know which one it was, where someone mentioned you should hold onto your dragon claw keys after using them because you will likely use them again further on in the game. So I thought it might be a good idea to start a thread of what not to sell where people can add items. Obviously we dont want spoilers, just name the item.

10 Fire Salts, 8 Common Soul gems, 10 ice wraith teeth, 5 flawless amethysts, 1 mamoth tusk, 1 golden ore. That's all I can think of at the moment, no spoilers just like you said.

Dragon Priest masks

Keep a stash of Nirnroot, Deathbell and Nightshade. You can just go out and collect them when you need them, but I reckon it would be less hassle if you just put one away for every two you find.

Bear pelts are in demand too.