You're more likely to sell it for between $250-300 (if it's in great condition), but okay...
If I get the base model iPhone 6 (16GB) on Next, I pay $32.50 a month, which is $390 after 12 months. That leaves $260 I don't have to pay when I upgrade to the next phone.
See? It's basically even, and I don't have to bother with even trying to sell my phone. It's a guaranteed trade-in value; no seller's fees or Craigslist spam.
----------
If you call and ask them to your carrier will put you on the discounted plan even if you're not actually using the financing program. I have been on AT&T's Mobile Share Value Plan since Next started even though I don't have a phone financed under Next yet (not until my 6+ arrives, anyway).