I've been using swappa for about the past 7 months selling and buying numerous smartphones. In the past I also went through Gazelle and/or craigslist. With Gazelle I always felt shortchanged but the money came instant, with craigslist, I dealt with the spam & the crazies. Swappa is a nice middle ground although just know that using swappa requires time, patience and good communication skills. Your products should always be showcased with good pictures, good description and honesty about it's conditions.
Another way to sell you mobile phone is to use a comparison website like Cellmefree or Sellcell. They select the most trusted professional phone buying companies to make sure you get the most money for the sale and the best experience (safe trade, free shipping, quick payment)