It is extremely simple in MP, so you may be over thinking it.
Find the listing under your sold or completed orders.
There will be a drop down box on the left [I think]. On that menu will be an option to refund. Click on that and follow the instructions.
FYI this form is a little tricky. The very FIRST box must be filled out before you are allowed to go further.
mam98031 • Volunteer Community Member • Buyer/Seller since 1999