You should always give measurements in your clothing listing.  Then this sort of mess would not happened.  If the buyer did not seem to want the item when they were picking it up, you should have just say fine, I will send you cancellation request, which will let you out of the obligation to buy.  End of story.  Why let cheap item problem escalate to this?

And no, eBay will not most likely remove the NEG, unless it is something abusive, against the rules.

Mutual cancellation does not remove the right of leaving FB.