Kingsway8605, that is incorrect. I've sold multiple phones that I had. I had a BB Curve years ago that I sold because I wanted to get something new. I replaced the phone on my account with an old phone I had laying around until I bought the replacement. Didn't pay any additional fees,a nd certainly no ETF because I didn't cancel the contract.
Additionally I sold a HTC PPC 6700. After I shattered the screen for the second time, resulting in no longer being insured by the insurance company I NEEDED to sell the phone. I sold it on eBay and the same day I put it in the mail I went out and bought a replacement at full retail. Again, NO fees, NO ETF, and NO issues with Sprint.
A "BAD" ESN only refers to phones that have been blacklisted as it has been discussed earlier.