This is a brilliant illustration of why I swore off Ebay once and for all back in 2009, their system and relationship with Paypal are absurd.
Why not send the refund in a separate transaction from the one you're having trouble receiving out of your available balance? Do you not have the amount?
I'd contact Ebay through their Help & Contact page to explain the situation to them including that you've discussed it with the buyer but they don't seem to be understanding.
Contact Paypal as well concerning the issue receiving the transaction. Presumably, either the buyer isn't familiar with using Paypal and thus unknowingly did not complete the transaction or it's an issue on Paypal's end.
I also advise you get your ass off Ebay, Amazon is far more reliable and efficient but that's up to you.