I always thought a 42 was a UK size 8 but not a 9 , as the buyer suggests. Perhaps send them a polite message to explain this , maybe a screenshot of say a Clarks shoes size chart?
End of the day, you have a No Returns, so you do not have to accept it for the reason ' Does not Fit'.
If you do accept it , you can of course, and to be honest sometimes it's best to, then just make sure the buyer knows they have to pay for that fully tracked return postage, and you only refund them, on receipt of your item.
@rosie1jean