They're pretty decent.
They hold their value better than most smartphones out there.
Especially if you dont have many scratches, dings or dents then you could sell fast.
For my 2 year old unlocked i6 Plus I could get about $400-420 right now and that's about 60% back so a 40% loss with 2 years of use.
I checked out this series last year before selling my phone. Article 1 uses data to predict how resale values are effected by the new release. Article 2 shows how resale value holds up a few weeks after the release of the new iPhone. Article 3 shows resale value after a 6 month period.