How to resell an iPhone 5s that is new and still under contract?

Just received a new iPhone 5s from AT&T from the mail. Under contract, so it is locked. I see plenty of auctions on eBay selling this, so I believe this is doable, but I'm not sure how exactly. I've read about first activating the phone, then removing the sim card that it came with it, then reselling it. Can someone confirm that? Or please provide me a list of things to do? And a list of things to keep in mind?

Take out the SIM and keep it.It'll work if you get another unlocked phone or AT&T phone.

In your ad on eBay, say it's an AT&T iPhone 5S, and it's not unlocked (so it will only work with AT&T), and there's no SIM card included.

Some buyers might also want to know if it has a clean IMEI (that's the unique ID of the phone). All phones have a clean ID until they are reported lost or stolen or have a delinquent account. So you could include the IMEI in your ad too, so people could check it with AT&T (or at least be ready to provide it to people who ask).

P.S. You don't have to tell AT&T anything and this won't change your contract. But if you need a new phone to activate on your current service, you won't be eligible for an upgrade/discounted phone, so you'll either have to pay full price for a new phone from AT&T, or get a used unlocked or AT&T phone. Then just put you SIM card in it and it should work fine.