Where can i sell my iphone with a contract that is not paid off?

The phone was with sprint

You could sell it on ebay put it as a bad esn

You can sell the phone on eBay, Craigslist, as well as any place that takes electronics for sale. As long as you remove the phone from the account before you sell it, and the account stays open until the contract is up, the ESN would not be bad, and as such the phone would work.

However, if you sell the phone and then cancel the contract without paying the ETF, the phone would then be blacklisted and as such would not work any longer.