How to sell massive, vintage and unsorted lego collection with manuals?
Depending on what you have, your best bet is probably to re-build each set, and sell them as complete.  Try BrickLink if you're not in a hurry to sell.  You can see how much sets are selling for, and price yours accordingly.  You can list them until they sell, and they only charge a small percentage when the items sell.  If you want fast cash, sell them on eBay.

These two sites will get you the most bang for your bricks.