The online platform (the name may or may not rhyme with 'beray') will not respond because, after waiting a week for the Seller (who so far has a 100% positive feedback rating) to respond, the sale is now over 90 days old
Call eBay customer service. Explain to them why it is longer than 90 days. Mention the date and time of the various emails if no longer visible (aged out). When told it is over 90 days, ASK TO THE MATTER TO BE ESCALATED, due to the timing circumstances. Keep pushing for the escalation until some human looks the matter up.
There is no issue with naming the seller here for the protection of other. Post the stamp, the certificate, screen shots of your emails received about winning the item, paying for the item and the like as the eBay information is gone.
One we know the seller there may be more leverage to be had for you.
By the way eBay runs eBay thus you and the seller cannot agree to actions not permitted by eBay, even if you, the buyer and seller agree while on the eBay platform.
Your title is, "What Recourse When A Seller
Will Not Honor Terms Of A Sale?"
An equally Correct title is, "What Recourse When A BUYER
Will Not Honor Terms Of A Sale as set by eBay?
That answer is easy: None is needed as eBay will protect the seller's interest when the buyer complains outside of eBay's contractual rules as agreed to by the buyer when registering to bid.
Harsh yes, but true nonetheless.