You can close your account through the My Profile section of your Dashboard.
Here are the instructions, along with the important warning:
Navigate to your user dropdown and select My Profile.
Scroll to the bottom of the page, where you will find the Permanently Delete Profile section.
Click the "Delete account" button.
A confirmation box will appear. Please read the warning carefully: Your account data, all associated ads, and your profile will be permanently deleted. You will not be able to recover your account or any listing history.
Confirm the deletion to close your account permanently.