The best you can hope for is that the buyer doesn't pay, even if they do you should refund them immediately with apologies

You should never list anything unless you are 100% sure it is authentic and if needed you can prove it is. I am no expert but if asked I would say it was fake!  As soon as you had any doubts you should have ended it.

It is not just against ebay rules it is actually against the law to sell counterfeit items and not knowing they are fake is no defense.

If you have sent a mutual cancellation request the buyer can refuse it in which case you will not get your FVF refunded, they can accept it in which case you will or they can do nothing and after a certain time, 7 days from memory, it will time out and you can close it and get the refund.