I would like to know if its free or how do i pay for it ?
Yes, posting sale items on craigslist is free. You need to work out payment (from the buyer) outside of the site however (get cash when you sell something).

You can also sell stuff for free on http://www.whyabe.com by registering as a supplier and creating a free online store.
For free complete guidelines in internet business. Check out my website.

Thanks and Good luck!