How much can you make? Probably nothing. As a used item you'll probably need to sell them for less then a new one. (unless you can find a sucker...) Lets make some numbers up. Lets say you bought them new for $400 each. You can sell them to a friend for $300 each. You can buy the new 780 for $450. To get on the bleeding edge new it should cost you $900, but you can recoup $600 of it by selling your old cards so it will really only cost you $300, or $150 per card. (just made up the prices, no clue how close any of that is.) As I mentioned the longer you wait, the more the used prices for the GTX680 will fall. Not everyone believes in this model. I personally believe in upgrading when I need to, or when I find a great deal.
As for where you can sell them you can always try ebay or your local Craigs list. There are also FS/FT (For Sale/For Trade) forums on popular websites. Toms, Anand has them, as does HardOCP which is my personal favorite. It helps to have heatware (look it up) along with any other online reference. Some sites like Hard also have post requirements. This helps weed out the scammers.