Where can I sell my iPhone 14 to buy an iPhone 15?

eBay, Craigslist, Amazon, private sale to someone you know.

You can also trade it in to Apple; you won’t get as much for it as in a private sale, but it is much less hassle→Apple Trade In - Apple

You could check out Apple Trade-In:

Apple Trade In - Apple

This is the simplest and easiest way to get the value of your iPhone 14 towards an iPhone 15.