How can I sell an elephant tusk?

You don’t. You burn it. If you don’t care about the animals, then you may care to know that this is illegal.

If you do have compassion toward animals, please read about elephants, watch some documentaries and then burn the body part that inhumane fools killed a soulful, endangered creature for. There is so much to be learned from elephants, and for God knows what reason, man has made ivory monetarily “valuable”. The means to obtain that tusk has caused severe emotional distress for an entire herd. That herd will visit the place where the elephant was killed, for years to come… they will grieve for the rest of their lives. That’s what elephants do. Elephants bonds are stronger than many human families.
The elephants in the photo are mourning the loss of one of their own. They refused to leave the body.