Kind of depends, where you are. In the US? Apple store/Apple.com. But with more discounts Amazon, B&H, and number of other resellers. To sell, there are trade in places where you can sell (google it) or Apple will take trade in (https://www.apple.com/shop/trade-in). Most resellers will not. Also, you can try to sell on your own...
In other parts of the world? Varies a lot and it may be difficult to get help here.