I NEVER trade in my phones. You can always get more by selling it yourself.
Theoretically that's true, but with unlike with any other used item, buying used tech is a real crap shoot without a 3rd party providing the buyer with some sort of insurance. I mean, when you resell clothes, toys or whatever else, you can see if there is wear and tear, but with tech, unless you get a chance to actually use it and put it through it's paces, you won't know if there's anything wrong. I'm hesitant to buy via Craigslist unless it's a HUGE discount due to this risk. This means that as a seller, you either discount the tech by more than usual, or go through a site and get a huge chunk taken away by the platform. When you think about it in terms of the amount of legwork you've got to do in order to sell your device, that may take more effort and time.
Case in point, I got a free pixel watch with my Pixel 8 Pro. I didn't want it so I sold it via Mercari... For $180. It was new, and the retail was $300, but no one was willing to buy it for even $250 because they didn't want to take the chance...that and everyone else was selling theirs for a loss too.
Tl;Dr it takes way more work and risk from the buyer to sell your used tech for a fair price on your own. Trade-ins offer a really good deal (most of the time) with way less legwork.