Gazelle.com would give you $45 for a perfect condition "like never been used" unit; a "good condition" unit is worth $40. Amazon.com allegedly offers the most of the buyback services, but it would probably only be $5-10 dollars more than Gazelle. Note that Amazon only issues store credit, not cash (like Gazelle and some of the other buyback services).
One hundred bucks from Best Buy (as store credit) is actually a pretty good trade-in offer, considering it's no questions asked, hassle-free.
Yours is two years old, so the battery has seen some use; at some point it will no longer hold a charge. It's also 3.5 year old technology. As an iPod, the 16GB model isn't so appealing in my mind; the iPhone 4 has a single-core CPU and the camera is pretty mediocre by today's standards.
You could use it as a standalone GPS navigation unit with built-in maps, but you wouldn't get live traffic reports which is one of the most compelling features of GPS navigation in 2013.
Personally, I'd ditch it, but that's just me...