You use your credits. If shipping is free theres now $ involved. If there are shipping cost it's up to the seller as to what payment types they except. I always do free U.S shipping and outside the country is paypal only.
Also its an auction you have to bid to try to win it, just like ebay, except there's no money involved. This is the best site ever!!! If you look around there are tons of way to get free credits, especially when your first getting started. When I signed up I started out after everything with like maybe 2 or 3000 credits. I just got my mom started up the other day and she started out with 15000. That's a lot!