Include details like these:

• Restocking fee:
  Specify if you will charge a restocking fee, and if so, what it would be.

• Postage and handling charges:
  Specify who will pay for return postage and handling—you or the buyer.

• Item condition:
  State the condition of the returned item that would be acceptable to you, (such as "unopened box" or "opened box with all original materials").