If you want the highest prices, you'll have to sell privately, ebay, amazon (not so private), craiglist, etc. Selling to a professional store such as Adorama, you'll only get 70% of the used price, whereas other places you can get more money. You're right though, the 5D Mark III price isn't going anywhere. However, if you think you'll suddenly have greater experiences and greater IQ changing to Nikon, you're in for big disappointment. If however, you are going for price, good luck to you. You may be able to purchase a Nikon camera at similar capability of the 5D Mark III and cheaper the price.