You can sell unsorted lego by the pound (on Ebay they will fetch ~$8/pound), if there are no minifigs or special bricks. If you have minifigs or special bricks, and are willing to pull them out and sell separately, you'll make much more.

Prices on Craigslist in higher cost areas tend to be higher, as usual. If you live in a higher cost area, buy on Ebay, sell on Craigslist.