I was just wondering if it is a good idea to sell the talisman or keep it?

Depends, keep it for now and see how your game pans out, certain things may or may not make the quest impossible without MODS :-D

well the guide mentions getting the tongue of the troll. I spared his life on the quest, so I guess I can't complete it unless I go back and kill him :(?

Yup fraid so,

You can let the troll live and still complete the quest. You need to win the troll head from Sendler as a prize in poker. But I don't think you can get it without mods if you do the poker quest before getter far enough into the troll quest.

Hmm guess I wondered if it is worth my while later in the game to have this medallion.
It kinda feels wrong to betray the troll for no apparant reason lol

Well I glances at the medalian and it didnt look to be anything special but as I say I only glanced.

I hope they change this in a patch. :/ I don't see why I wouldn't be able to play him for the troll head after I beat him in the tournament or, hell, just threaten/jedi mind trick him. Definitely not going to betray the troll now so I guess this quest is unfinishable for me.

The medallion is useless, anyway. It's not important.

Everybody just likes to complete all quests completely.

There are other ways to get the trolls tongue.

Ther are not other ways to get the essence of death the wraiths drop, however, so be careful you dont use them in a potion. Same with the tongue.

I found paying attention to ingredients for potions paramount, you can easily use a quest item that you cant get back. Speciially when making many potions (I kind of spam Y / A Y /A)