Swappa gives buyers the option to use Paypal without an account:
Buyers can check out as a PayPal guest using a credit or debit card by clicking the PayPal button and then clicking Pay with Debit or Credit card
Based on my experiences of shopping on Swappa, it’s up to the seller to enable this option. I’m not sure if this is a common option on other online marketplaces/storefronts though.