Hi. Ok, first the seller cannot cancel the unless they send you a cancel transaction & you either click link to agree or if you do nothing for 7 days. If they sent that notification you can check link to disagree & the seller will be charged ebay fees on the transaction.
All buyers have 100% feedbacks unless they also are sellers. Sellers can ONLY leave positive feedback or no feedbacks. So a 100% positive buyer feedback really is meaningless.
But eBay cannot force a seller to ship any more than they can force a buyer to pay.
If you have been refunded you need to calmly give a negative feedback stating only the facts - Won & paid & seller would not ship unless I paid more. Or something to that effect. At the same time give the seller a 1 rating on all of the DSR star ratings. If they get several of these then ebay will take notice & sanction the seller.
Just do the above & move on & find a better seller.
Good lu