I’m sorry to hear that you had to go through that. I had a similar situation in the past. Sent a really good iPhone 7 (not mint) in and received it back with a broken camera and $0 value. Since then, I’ve always gone to the store directly.
Even then it’s hit or miss.
When trading in my iPhone X this year, first employee was a jerk who really pushed the screen down and concluded that the battery is swollen so will give only $100. When I said the battery replacement cost is $75 or so should give me 325ish, he said no. I left and took to another store and the guy said nothing is wrong with the phone and gave the full $400.