Well....
If you go to your My Account page and look at the top right-hand side you'll see an Close My Account link.
There is a process to it, however. Some minimum waiting period to ensure all transactions are concluded before they let you go completely. As I understand it, my account is still active so I cannot share with you any first-hand experience.
May I ask what led to this decision? Are you a buyer or seller? Or combination of both?