Very few sellers can take credit cards directly. And most buyers are loath to give their credit card information to strangers over the internet.
Paypal gives a neutral interface for both buyers and sellers. There is no charge to buyers to use the service and if you back your Paypal account with a credit card, your payments will be instant and you will have two levels of protection against possibly unsatisfactory transactions, PP's Buyer Protection program and your card's chargeback program.