"Similarly upon cancellation after a few minutes of purchase by the buyer I have been asked to accept (which I accepted) the cancellation and pay the user by your credit card".
You shouldn't have to pay anything via "your credit card"if the cancellation took place soon after the buyer paid for the item. The transaction would be null and void as no money has been transfered as of yet.
If the buyer wanted to cancel after a few days of purchase then eBay would withdrawal the funds back from your chequing account linked to your Managed Payments. If there wasn't enough funds in that account then they would withdrawal the funds from your credit card you have on file.