This is such a terrible thing to happen, and it can test your  trust for customers in future.

I myself learnt the hard way and from now on, as a seller, i actually won't 't leave feedback until i have recived it.

At first i just did it without explaining, then i had a customer who actually thretened that if i didn't leave feedback first i would get reported, so i did just to get him to stop hasstling me, and he never left me feedback. So from now on i send a little thank you that explains that feedback is left once it is received. That way i feel it's fair game.