Q: Can I sell a French Iphone in an American Apple Store and get a new Iphone 6 that will work in France as well?

Hi everybody!

I have an iPhone 5 and I want to take advantage of my next trip to the US to get an iPhone 6 Plus.

If I go to the Apple Store, will they take my iPhone 5 back (I bought it in France)? If yes, any idea for how much?

Will the iPhone 6 work without any problem when I'll be back to France?