I don't know how much things have changed, since I have a regular old iPhone 4, but the first thing you should do is take pictures of your phones while they're working. Take good front and back images. Then look at auctions for phones that have bids [http://www.ebay.com/itm/Apple-iPhone-5-32GB-White-AT-T-/301492369663?pt=LH_DefaultDomain_0&hash=item46325878ff&autorefresh=true] on them. Notice their descriptions regarding which carrier they're tied to (if any), which accessories are included, etc.
Even though you won't be listing the following data, you should record your phone's information, such as the IMEI and serial number, which can be found under SETTINGS-->GENERAL. This is for any potential problems down the line after you sell them.
If you're ready to wipe your phone, while you're still in the GENERAL portion of the menu, scroll all the way down and you'll see RESET. Hit that. Then select Erase All Content and Settings [http://support.apple.com/en-us/HT201274]. The Apple site brings up a good point, which is to keep your phone plugged in and charging while the reset takes place as it may take some hours.
Remove your sim card, the new user won't need it.
As far as selling online, I've only some moderate experience with eBay through a recent job, but never for personal use. If you have a decent selling history, you should dictate your terms within your description. State that you reserve the right to deny any sale to anyone under a minimum number of auctions, like 50 or 200, whichever you prefer. Terminology like "Auction includes only what is pictured here" work fine. Just don't rush through it. You can save your partially completed auctions and finish them later.
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You can also just save yourself the time and check the big companies for trade-in values, but you might not get as much as you would if you sold it yourself.
Amazon [http://www.amazon.com/gp/search/s/ref=tradeinavs?url=rh%3Dn%3A9187220011%26i%3Dtradein-aps&field-keywords=iphone&Go=Search]
Apple [http://store.apple.com/us/iphone/reuse-and-recycling] - Credit towards new purchases only, I think.
Best Buy [http://www.bestbuy.com/site/electronics-promotions/online-trade-in/pcmcat133600050011.c?id=pcmcat133600050011]