Craigslist.org is like an online Pennysaver that you can check every day to find new ads for things you may need or want. I had listed a house for rent last year on craigslist and my ad was copied and used by an offshore scam ring to solicit deposits and rent checks from unsuspecting prospective tenants. I had to immediately remove the listing and post the home for rent in the newspaper.
There are some scammers lurking there but, for the most part, if you want or need to sell something this is a great place to go. I would suggest that you always have someone with you when you either go to purchase or have someone come to your destination to purchase from you, for safety measures.
With any second hand purchase you run the risk of purchasing something procured in an illegal manner but your own personal filter and personal choice can work that out.
All in all, it is a great place to go for a used product and, sometimes discounted new products.