First things first. Why are you reading this, You should never under any circumstances sell your LEGO collection. Unless you're selling it to me, or you need the money. In which case, I am sad for you, and hopefully happy for me.
For now, let's assume that you need the money, and so are not selling it to me. I'm sad, for you and me, but I still want to help. So now, how to proceed? Well, it depends, there are really 4 ways to proceed. You can either sell your LEGO as a bulk lot or part it out and sell the parts individually to those builders who need more of your particular part.
Before we discuss your LEGO sale options, it is important to realize, real genuine 100% LEGO sells. NONE of that other stuff does. If you're trying to sell your Mega Block or Kreo collection, I wish you luck, but you are probably better off donating it to the thrift store, or the landfill. While the why, is a subject for another post, let me just say, that I've never seen a Mega Block Users Group, and I think there is a reason for that.
Selling as a bulk lot is the easiest, but generally has the lowest return on your investment. Here too you have a few options. You can sell it on eBay, where you are likely to get about $7/pound if it's in good shape, and you can demonstrate such. Alternatively, you could sell it to your friendly Adult LEGO fan. This may or may not get you a better price, it will depend on what specific pieces are in your lot. These are both great options if you need money quickly, and don't want to spend a lot of time preparing your LEGO for Sale.
If however, Money is the primary goal, Then you should definitely consider selling your parts individually. Be forewarned, this takes significant time and effort.
You will need to sort your parts-by-part and color. After you have done so, you can sell them on eBay, or Bricklink. I personally recommend eBay to get the best price out of mini-figs and Bricklink for the rest. When selling parts individually, the price ranges drastically by part, and color, and not by size as you might expect. Common parts can sell for less than a penny each, with some rare part color combinations exceeding the $1 each. Minifigs follow a similar pattern. Common and unlicensed figs can be relatively inexpensive, at $1 or less, while the rare and licensed figs commonly sell for $8, and the most expensive figs (promotional and Comic-Con Figs) can easily exceed hundreds of dollars.