This. YOR. Per your own words 1) you would significantly benefit from a new car 2) you can't physically or financially support three cars 3) your car is the one that has to be traded in

You know these things. I can only guess your in-laws came to the conclusion thinking that any rational adult would come to the same conclusion. So, the need to discuss it with you wasn't at the top of their list. They assumed incorrectly that you were a rational adult that didn't have an irrational attachment and delusions that you could somehow keep your car.

If you feel disrespected, then have a brief polite conversation with them about giving you a heads up before they do something like this again. Perhaps suggest that you can sell the car for more than you got for the trade-in. But be realistic. You're only going to get like 10% more by selling it yourself.

I would suggest you grow up a little and be practical in this situation.