If the buyer has not asked to cancel and you cancel the order regardless then you could potentially end up as the one with the black mark on his/her account, as if the buyer were to contact eBay and say "I didn't ask the seller to cancel the sale - I intended to pay for it" then eBay may well hit your account with a selling defect for cancelling the transaction without the buyer's consent. If, on the other hand, at least four working days have passed since the buyer committed to buying the item then you ought to have an option available to cancel the sale on the basis that the buyer has failed to pay for the item. If that option is available to you then select it, as the sale will automatically be cancelled and the buyer's account will end up being hit with a non-payment strike.
If you have not already done so by the time you read this response, and you are consider relisting the item for sale, then add the buyer's User ID to your Blocked Bidders List, ensuring that you activate the option where it says "Don't Allow Blocked Buyers To Contact Me" so as to prevent any further contact from the buyer. However, given that you have recently been in a transaction with the buyer the blocks will not kick in until forty-eight hours have passed from the time that you added his or her User ID to your Blocked Bidders List. Once this time has passed, though, you can then relist the item for sale again and the timewaster would not be able to purchase anything else that you list for sale at any point in the future, unless for some reason you changed your mind and chose to remove the buyer's User ID from your Blocked Bidders List.
So, in summary, if the buyer still hasn't paid four days after the date of the sale and you select the option to cancel the sale due to non-payment, the buyer's account will be hit with a non-payment strike but your own account would not be subject to receiving a black mark from eBay for cancelling the sale.
