No need for a black market, there's plenty of public ones (Backmarket, eBay, etc.). That being said $200 seems not terrible given the step change in performance since then (I own a 2019 MBP and think we were very unlucky with our purchase timing). Backmarket seems to sell yours for ~$350-500, so maybe you'll get a little bit more trade-in for it.

I owned a 2014 MBP (~$1200?) for a long time and as late as 2019 it was resellable for $500.

It’s a casaulty of the Apple Silicon transition. The Intel Mac’s are not worth much

There's a legal second hand market where prices are obviously way higher than whatever Apple is offering for trade in.