Craigslist doesn't cost anything to post. They make their money from Help Wanted/Job Postings.
It's basically just like a free, online classified ad but you also get to add pictures.
If you want to sell your stuff, put pictures and explain it well (you're the salesperson).
You're going to have to allow the people into your house to look at the stuff, so I'd be careful. I've bought and sold lost of stuff, but there are crazy people out there. If you can, make sure that another adult is home with you when you meet the potential buyer. If it's something smaller, I usually try to meet in a busy parking lot where lots of people can see me.
Take care of yourself. It would be better to donate the stuff and stay safe than to try to sell it and not protect yourself.
You can also post on eBay, but you'll still have to allow the buyer to come to your house. Also, some people may want to come and look at it before they buy on eBay. Also, eBay charges a fee when you list an item (even if it doesn't sell).