So, this has happened many times before, according to the Ebay forums. An example from 2015:
https://community.ebay.com/t5/Archive-Bidding-Buying/Buy-It-Now-Paypal-asking-to-add-card/td-p/24701...
I have a PayPal account that's been verified since 1999. That was the same time that I created my Ebay account. I do NOT use a credit/debit card on PayPal. I have it linked to a bank account used specifically for PayPal purchases.
When I select Buy It Now and go to the payment selection screen, I have PayPal selected by default. Typically, I just click Commit and pay, but it's grayed out. There's a button under the PayPal selection directing me to continue to PayPal to complete the payment, which happens from time to time. It opens a window and I log in to PayPal. Except that now, a new screen tells me to add a credit card.
After calling Ebay, PayPal, and contacting the seller, I finally figured out what the issue was.
*POSSIBLE FIX*
(This was my fix, it may be yours, too)
My bank account had somehow become Unverified. I called PayPal back to explain and the CSR re-verified my account. Now, I can use Buy It Now with no problems.
