I'm setting up a marketplace app where people can buy and sell goods while my application takes a commission. I'm ok using PayPal or Google Checkout/Wallet to accept payments from customers but I don't get how to pay the sellers their money.
Is there a way to credit user's credit card or do I need all users to have PayPal accounts in order for this to work?
You're going to have a huge headache paying into a credit/debit account. The best way is to either use paypal (and automate the return revenue to them) or also support (i'd do both) a checking account option. Either way, you're paying overhead. Either way merchants and customers are going to be weary until your brand is trusted. You will also need good SSL certs.
See also
http://ptmoney.com/how-to-make-a-deposit-at-an-online-checking-account/