On more than one occasion, cancelled orders on eBay resulted in the loss of a great Value Coupon. Since these coupons are rare, and come across you only once in a while i would like to suggest, that each time the Seller is unable to ship an item once the payment has been made, amount equivalent to the "GIFT Coupon" needs to be credited back to the Buyer. At the cost of eBay or the seller is at your discretion.
Also, given the opportunity to participate in eBay's EDGE point's program, after having earned points and then a GIFT VOUCHER; the sales person tricked me by delaying shipment by a month. In the end, the order was cancelled due to non-availability and I lost all my hard earned points, since the program came to an end.
In case of such incidents, Customer Experience at eBay can be improved if something of Value can be returned - which i feel eBay has been neglecting.