Two things I drew from this post (which came via this Cult of Mac post):
The Samsung Galaxy S9 is a pretty horrible investment, dropping about 60% in nine months, as compared to the iPhone X, which dropped about 30% in the same span.
The worst time to trade in your iPhone is in the 3 months following the September iPhone event. This from the headline linked article:
A massive 68.86% of the iPhone X and 8’s total yearly depreciation was seen in Q4 following the Sept 2018 Apple Keynote (31.14% of their value between Q1-Q3). The months after the keynote is a trade-in black hole consumers should avoid.
iPhone values trend upwards every January. Lots more info in the article. Interesting.