Swappa and Craigslist are my usual go-to sources. I've tried to sell on here but people tend to be cheap and low-ball on here so I'm giving up on that idea.
On swappa they do an IMEI check before they allow you to list it.
Or the potential buyer can ask you for it also.
Pretty easy to sell on those places and with less worries.
Sold one on Swappa years ago. I prefer to meet in person as it yields a better market price. Craigslist is useful if you know how to protect yourself from a scam.
Edit: Just realized this is an old thread, but the advice is applicable.