If you want to buy a product but you only have a credit card to pay and the seller only accepts PayPal as the payment method, don't worry -- you can still buy.
Paypal does open up a PayPal account automatically for you when entering your credit card on PayPal page. However, you do not need to use the account if you don't want to. Simply treat PayPal as a credit card processor.
If you don't have a PayPal account yet, while on the PayPal pre-approvement page simply choose "Pay with a credit or debit card" option.
If you already have a PayPal account but don't have enough balance to cover the payment, you need to link your card first through PayPal dashboard. After you login to your PayPal account choose this menu "Money > Add a new card".
Once you've added your card, then on the checkout page you should be able to select your credit card as secondary funding source by clicking "Change" button on the PayPal checkout page.
Note: PayPal will use the secondary funding source you specify on the checkout page only if you don't have PayPal balance.