Cant cancel order

so I purchased something and straight after placing order I realised that the payment method is wrong so I straight away asked to cancel the order but the sellers instead of cancelling the order they put it as dispatched which obviously is impossible to dispatch the item same time as I placed the order and because of that I can't cancel it so I have to wait till the item arrives just to return it which will cost me and then order it again absolute waste of my time and my money I think I will not be using eBay again!. Also when trying to contact the seller it won't allow me to message really frustrating eBay gone down hill in my opinion.