Can I sell an iPhone before it's paid off?

I'm looking to upgrade my phone; I'm currently financing an iPhone 7 with AT&T with 16 out of 30 payments already made ($274 to pay it all off) I tried to trade-in with AT&T and they only offered $97, which I know I can get way more than that. Is there any way I could sell the phone for maybe ... show more I'm looking to upgrade my phone; I'm currently financing an iPhone 7 with AT&T with 16 out of 30 payments already made ($274 to pay it all off) I tried to trade-in with AT&T and they only offered $97, which I know I can get way more than that.

Is there any way I could sell the phone for maybe $500, then use that to pay off the $274 that's due? If I do that, will the IMEI/ESN still be good and activated with other carriers? Will I still be eligible for an upgrade?
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