Did you do this when you set it up?
To link your PayPal and eBay accounts:
Click My eBay at the top of most eBay pages.
Click the Account tab.
Click the “PayPal Account” link on the left side of the page.
Click the Link My PayPal Account button.
You'll be asked to log into PayPal to finish linking your accounts.
If so then it is linked.
And yes, your credit card will be charged if you do not have the funds in your Paypal.