As you quite rightly said that only you and the seller know the truth.
All of the responses have been made by what we know which is simply your respective posts and of course the photos.
The sellers pics of the strap look absolutely fine whilst yours show a strap which is complete goosed, look at that simple point and then try to understand the negative responses you’ve received?
Anyway good luck with selling or buying anything off here ( if you ever have ) in the future, I can’t imagine it’ll go that well.
Question to the OP- do you actually lose the money that you received or does the money come back from eBay direct and not your account?
FFF