PayPal recognizes credit cards it already has on file.  Through that link, PayPal also recognizes the account, and whether or not there are funds available in the PayPal account.

The application is written to use PayPal's Website Payments Standard, passing the transaction to PayPal for payment processing.  In that processing scheme, PayPal steers the payment to come from any available PayPal account balance first.  To get around it without touching the PayPal balance, a different (previously unknown to PayPal) credit card (and maybe a different email address) would need to be used. 

If the PayPal balance were zero (which occurs after you initiate a fee-free transfer of the balance from PayPal to a bank account), then it is possible to have PayPal charge the on-file credit card.  If you have a basic PayPal account, the transfers to your bank account may be capped at around $500/month.  If you upgrade the account to "Premier" level, there's no cap on the transfer amount, and there's no charge for this level of account.  The big difference between the basic and "Premier" level is that there's no fee for receiving money at the basic level, and you cannot accept credit cards for payments.  At the "Premier" level, credit cards are enabled, and PayPal charges $0.30 + 2.9% of the total.  Many eBay sellers cover this by factoring it in to the shipping & handling amount; many online merchants cover the cost via an online convenience fee.