A 16GB 5S when the 6 comes out will likely sell for around $300 give or take 10-15% depending on your area. I find rural areas tend to have higher resale values than metropolitan for obvious reasons (if it isn't obvious it has to do with population I would think).
If you are very worried about resale value, I would try and buy used from a trustable source. Used phones are already going for about $500 where I live (with a few hopefuls listing for $600), so prices are already well below retail.
I actually like to do craigslist, and have a few tips to help safeguard yourself from scams. Nothing is fool proof, but these tips are generally helpful.
1. Go with someone who has a box with matching serial number. A used phone with the box is likely not a stolen phone. Stealing both the phone AND the box is much tougher to do than just the phone. An original receipt is AMAZING, but that is going to be a lot harder to get. You can try and ask if the person bought it at Apple. If so, the receipt will be easier to track as it likely linked to their email.
2. ALWAYS make sure you are able to set up the phone as new ON THE SPOT. iCloud locked phones can not be recovered unless you get the info from the original user. There is no exception to this. NONE. If your phone you just bought is iCloud locked and the seller is long gone and not responding to your communications you now have a paperweight (and likely a stolen phone).
3. Meet at the carrier company store (not an authorized reseller) and have them check the phone out for you (simple run of IMEI and ESN). Also have them move the phone to your account. Make sure the person selling you the phone has authorization to make changes to the account the phone is currently on. Again, this step helps reduce the risk of a phone being stolen.
