Errand for Tonmerk
 

Serna Tasknon will take the shark bones and 10 gold and not return the shark bone powder. I did this with a level 25 Halfling Druid that is amiable with her. I have done the first part of the Sentry Xyrin quest and recieved the Potion of Marr so I believe I have enough faction for the shark bone quest.

I successfully completed this quest without issue.

Either you didn't turn in the correct items/gold, or the trade didn't process because you were invisible etc.