Radish,You are in a tough position.eBay will only act as an...

Radish,

You are in a tough position.
eBay will only act as an intermediary between Buyer and Seller,a bit like a dating service.
They will tell you that they cannot force the deal to be consumated.
They will through their dispute resolution centre contact the Seller and ask them why they will not conclude the deal but cannot force their hand.
I have used Negative feedback as a leverage.
If the seller has a reputation they want to protect you may choose to leave very disparaging feedback on the sale.
This cannot be removed by eBay.
This may in turn prompt the Seller to enter negotiation's for the removal of the neg feedback by entering mediation with Square Trade.
If that does not entice him out of his sleepy hollow you are not left with many options.
You can get Sellers phone number and talk direct but as he has already advised you by email that he won't sell you will probably not sway him.
If he is in Australia there is a slim chance that you can make a complaint under the TPA (Trade Practices Act )
for misleading and deceptive conduct Section 84 from memory,but very expensive to litigate.

My best advice is assume you have lost the item,think of the most disparaging remarks you can enter into feedback with 80 words or less,and if he is a seller with a history to protect he will have to pay $ 20 USD to Square Trade to enter mediation as that is the only way negative feedback can be removed from his user ID.

Hope you get a positive outcome.