Buyer has not paid for an item he purchased from me 6 days ago.

[robertclassiccoinsandstamps]

How can I cancel that purchase and receive my seller fees returned?

Item # was   

 115169420563

[wooden_flower]

Go to your sold items page and select cancel transaction next to that item.  Then select buyer hasn't paid.

Most fees are not charged until the buyer pays, so there probably isn't any fee to be refunded.  Insertion fees are not refundable.