Just to clarify a few things: You cannot fight Kakrak before talking to the princess and accepting the quest, because only then the lever for activating the bridge (which leads to Kakrak) can be activated. Thus, you cannot get the pendant without having the quest active. Once you got the quest, fought Kakrak and collected the pendant, the latter cannot be sold.
You can easily check whether you already got the pendant or not by looking at your journal. If it still says that you should retrieve the pendant, than you never took it. However, if it says that you obtained the pendant, than you got it at some point and may have stored it in one of your vaults or chests.
There seems to be one scenario which might explain your issue: You got the quest, stumbled upon Kakrak without noticing it, killed him, but never looked at his loot. In this case you don't get any warning message about leaving quest items behind and Kakrak will never spawn again, rendering the quest impossible for you.
You can easily check whether you already got the pendant or not by looking at your journal. If it still says that you should retrieve the pendant, than you never took it. However, if it says that you obtained the pendant, than you got it at some point and may have stored it in one of your vaults or chests.
There seems to be one scenario which might explain your issue: You got the quest, stumbled upon Kakrak without noticing it, killed him, but never looked at his loot. In this case you don't get any warning message about leaving quest items behind and Kakrak will never spawn again, rendering the quest impossible for you.