If you are familiar with ebay, that would be a good route. there are tons of lenses on there and if you lst it correctly it can be easy. BUt to be honest im suprised your not using either of your other lenses, only your 70-200? well if not then i would use ebay becasue i have used ebay before to buy and sell used equipment. I have never been let down and im sure you wont. The other option is B&H, they have a trade in department and its kinda like a safe option if you cannot sell it any other way. I think you could get more on ebay however. You can get a quote from B&H and see how much they will give you before ebay however. I hope this helped