You can certainly use the link Bob Timmons provided to make such a suggestion, but I wouldn't honestly expect it to get much traction at Apple. They really aren't in the business of helping users sell their phones. And they already have a simple way for you to see what they'd give you in trade for your phone --> Apple Trade In - Apple
Honestly, think about it. Apple is in the business to sell products. What would be in it for Apple as a business to build such an app?
Plus there are other sellers who sell used iPhones, like gazelle.com you could check there to see what they're selling a model like yours for.
If you choose to sell your iPhone yourself, make sure you keep this link about what to do before you sell your phone handy --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support