Can customers pay with PayPal across all merchants in my marketplace even if some merchants don't have PayPal accounts?

Yes, customers can pay with PayPal across your marketplace, even if all of your sellers don’t have an existing PayPal account (they can get one after payment).

You can onboard your sellers to PayPal after a customer makes a payment. See Onboard After Payment for instructions. You can also define the type of PayPal account you allow your seller to have. See Onboarding Checklist -Intent for a list of account types.