I see your point but if payment was in PayPal and refunded in PayPal a case couldn't be opened or if it is, as soon as the seller informs of a refund, the case will close in sellers favour.

If a cancellation request has no response for 7 days it can be closed by the seller.

However I believe the new system now allows a seller to cancel a transaction without needing the buyer to agree.

(I personally have not done one)

I agree, if the seller is not in hurry to sell the item and from a polite, caring standpoint, I also would give the buyer plenty of time to respond, but don't think the seller is under any eBay policy, legal or moral conditions to do more than refund after no response within a reasonable period of time.