One plus generally has good resale value, you can easily get much more than that selling on swappa or Craigslist. See what's the going price on swappa, price it for $20 or so less than that if you want a quick sale.
Obviously having the original box, all accessories, good condition phone can help your phone sell quickly compared to a beaten up phone.
I'm not saying trade in is bad, I do get the easy part but I have seen lots of companies handle trade in very badly as well. Unless you are trading in locally in a store, in my personal opinion selling independently is better if your are anyway planning to ship.