I have been impressed with
Zen-cart. It is free open-source software with very good functionality and easy installation. There is an active user forum and plenty of MODS and template packs to customize the cart.
Accepting credit cards directly will mean a merchent account and will need to be purchased, not an inexpensive way to go. Otherwise, there are plenty of services out there to help you handle credit card transactions (PayPal, Authorize net, etc.)
Good Luck,
Ray