You can sell your old smart phone on eBay.
I sold two Treo 650s for about a third of what I had paid after a year worth of use.
You can expect Nigerian scams out of England and little old ladies trying to buy your phone for their minister sons who are heading to Africa to save the natives from damnation. Ignore both and you’ll do fine. Limit auctions to residents in your home country. Also call your phone provider and make sure they clear your ESN number before you list your auction.
Or you can donate your phone. WirelessRecycling.com will find drop off and charities near you.