iPhone trade in values - to trade phones back to Apple themselves - fluctuate over time, and eventually they no longer accept certain kinds of phones. You can still trade in an iPhone 7 - released in 2016, about 6 years ago as of this writing. But no longer does an iPhone 6 have value back to Apple.

This tracks those values - it runs every month and records prices to iphoneTradeInValues, which I'll visualize once there's some data.