You'll maximize your profit by selling it on eBay. They are going for as much as $475 right now if you include insurance and shipping charges in the sale price. If you sell it on eBay, only ship to the verified PayPal address. Craigslist is another great option. You'll probably only get half the actual value if you go through Radio Shack, but it's much more convenient.
You'll get even less if you go through an online pawnshop like gazelle, so I don't recommend that route. They tend to drop their offers after they receive your phone and then ask if you'd rather have it shipped back to you.