You set the default with the above link (get to it via My eBay: Account: Payments.
You can change from the default payment method (and shipping address) when you make the offer on the confirm screen when you offer/bid and it requires it by clicking the appropriate "Change" link.
If you choose PayPal you can set the funding method for PayPal to use for such authorizations at paypal.com: Settings: Payments: Automatic Payments (scroll down the list in the left column, hitting the button for more entries as needed, until you find eBay, click it, and edit in the main column.
