Hi LizzyB,
If you posted an item to eBay without a return policy, you should be able to go in and add one provided that the item has not received bids and has more than 12 hours left. In order to do this, you would need to log-in to your eBay account, click the "Revise your Item" link on the actual eBay auction, and then update the "return policy" field as desired.
If you have any additional questions or concerns, please feel free to contact our customer support team using the web form on the following page:
http://www.auctiva.com/help/requesthelp.aspx-Mike