Best Buy will take your old consoles and give you a gift card to use at one of its bajillion locations. The process is fairly straightforward and shipping costs are taken care of by Best Buy, not you.
You provide some basic info about the current shape of your console, print the shipping label, and send it off. About a week after the item arrives at the shipping center, you’ll get a Best Buy gift card.
My mint-condition Nintendo DS (mint condition because I think I’ve used it five times) is worth $46.01 according to the Dealtree.com-powered trade-in center. It’s the old chunky DS, too. Not bad, eh?