If you receive the below error message you will need to make sure you have the correct return policy set in your listing template.
'Please fix the listing error: Return policy value is not valid. Please specify a valid return policy.'
To correct this please select settings-> channel integration-> edit eBay-> listing profiles-> edit the relevant listing template-> 'shipping & payment' tab-> enter the information under the 'return policy details'
Once you have entered the correct return policy in your listing template, please try to list your product again.