I personally think he dumped her at sea and they never found her. If she had been sold into sex slavery, couldn’t they have ransomed her for a lot of money?
Not without exposing what Van der Sloot had done and the outrage here in the U.S. that would have followed and who knows what the U.S. government or the Venezuelan government would have done to the perpetrators. They would have been put out of business and faced severe penalties. I imagine their silence was part of the deal.