If the buyer has opened a Not As Described dispute it makes no difference if you have proof of purchase.  If the tickets are not as described the buyer will have to return them to you by registered post in order to get their refund.

You could ring paypal on their Free Call number and plead your case but it is not a given that they can or will help you.

I have never gone through a dispute where time is as crucial as this, but you can but try.

Just be very nice to them...you do want them to help you and getting angry will not help matters at all.