I have found that I end up making more money selling my iphone on ebay (even after all the fees) than sending it to one of these trade in places.
I did have a cheap Samsung phone that I had no use for, so I sent it in to one of these trade in companies for $11. After 3 weeks I hadn't heard anything so I checked the tracking and it said they had received it 2 weeks before. So I called them, and they told me they never received it and they said the tracking is meaningless because when UPS delivers them 1,000 phone boxes, they don't actually scan each box, they just write delivered for the whole thing. It was only $11 so I wasn't going to go crazy over it, but my point is if you were sending in an expensive phone, I don't know who would be responsible or if there would be a way to get your money back if it gets lost.