Those are the rules, a seller does not have to accept a cancellation request.
The item has not been physically dispatched but when a seller triggers a Simple Delivery postage label, it automatically marks the item as dispatched. Don't blame the seller for this, is an eBay quirk with SD.
What do you mean by ' payment method wrong' ? Did you have an invalid credit card on your eBay account, then the payment will not go through.
@steponavicius20