Like others have said, Fredmiranda.com or ebay are the best bets for making a lot of money, I've had no experience with KEH.
At fredmiranda.com you have to pay a small fee though (but substantially less than ebay, maybe like $4.95 in compared to $20+ depending on your final price for ebay)
I sold my old DSLR gear which was the Digital Rebel kit for far more than I expected, sold it for $400 on ebay with a buy it now, I didn't expect it but someone bought it. That made the most money and I bought a Rebel XT for the same price I sold the Digital Rebel, not so bad trade off. Fredmiranda is probably the safest (you set the price) and best site though if you want a quick response, very friendly people and awesome prices to buy used but most likely you'll make the most on Ebay (maybe).