Adding a debit card to your PayPal account will not result in the transfer of funds to your PayPal account.
You do not need to have funds in your PayPal account to send a payment... just click on the eBay "Pay Now" button or the PayPal "Send Money" tab to send your payment. The payment amount will be deducted directly from your linked debit card account.
Your Mastercard Debit card is tied to your bank, not PayPal. That's a good thing too. PayPal has been sued for freezing people's PayPal accounts ( that have no disputes and are in good standing ) which is illegal. As is already mentioned on another reply, PayPal will do an electronic transfer from your bank to the seller on any purchase you make.
