Backpage is a free classified site online just like Craigslist. You can earn money on this site through Buy and Sell, Selling your work, and Freelance Work you can just follow the same strategy I've posted before for Craigslist here.
If you are selling stuff, it would be better to use a lot of free classified site online the more your ads are the more chances that it will be seen, and sold. When I was selling an Anti-virus, I use a lot of classified Ads and it really pays off instead of posting it on a single site.
Just like what I've said before be careful in dealing with strangers, there are a lot of creeps out there.
You can check their site here Backpage.com