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@ronteal wrote:

Without going into details I believe that a buyer has committed a return fraud and I have been advised to report it to Action Fraud. Has anyone done this and had the refund successfully reversed? 

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When a seller is in the process of being defrauded - i.e. the buyer has returned a different item - the process is to report the buyer to Report Fraud and obtain a reference number. In the return case there is a link that specifically states "Report a problem" that appears once the return tracking indicates delivery or attempted delivery. The seller clicks that link and selects "Returned a different item" (or whatever the most accurate reason is) and provides the reference number and report obtained from Report Fraud. This typically results in eBay refunding the buyer out of their own pocket with no impact to the seller.

As your case has now presumably been closed in the buyer's favour you have one chance to appeal the outcome. I'd suggest doing as the customer service agent advised and file the report with Report Fraud providing the reference number and report they give you in your appeal. There have been a few reports on these boards of sellers being reimbursed by eBay after appealing a closed return case in this manner.