How much should I sell my 4S for?

So I got my 6+ in the mail on Friday (best smartphone I've ever had btw) and I'm ready to sell my 4S. It's a 16GB Verizon model in black in excellent condition - everything works fine. I'm seeing them sell for around $100 or so on eBay. And GameStop is offering me $99 for mine. Do you think it's worth $200 or should I just sell it for $100 cash to Gamestop?

Also I have another 4S I'm selling but the whole back of it is cracked from a drop but it functions fine. GameStop said they'll take it for $40 cash. That seems too low for a functioning 4S or should I do it?

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Or would it be better to sell it to Verizon for $200? But if I do that, I'm pretty sure they'll only give me credit back and not cash?

 

Yea I'm still with Verizon but I don't think Verizon offers cash for trade ins, just store credit.

And I'm on a family plan with my family. I'd rather just get cash and use it for my car as it needs maintenance.

 

Can I just walk into a Verizon store and give them the phone for $200 or do I have to mail it in? If I can just walk in then I'll probably go that route.

Also does anyone know how much I could get from Verizon for a 4S with a cracked back? Everything works fine but there are cracks in the back because of a drop.