The usual way forward in these situations is that the seller accepts the return and gets their item back, if it isn’t their item they immediately file a report with Reportfraud (used to be Actionfraud) and get a report number:
https://www.reportfraud.police.uk/
Then click the "Report a problem" link in the eBay case, select "Different item returned" as the reason and provide the Reportfraud reference number and report. This stops the automated returns process that would ultimately force the seller to issue a refund.
Then, someone at eBay reviews the case. As to what happens next depends on a few factors - if the buyer has done this before, if the seller is a long standing eBay member who's had no issues like this before etc. We've seen a good few threads recently here in which the sellers have been in a similar scenario but have not lost out.