Man that "held temp fine" line gets me every time because it's always the compressor hiding the truth. Sorry you got stuck with that, $400 plus a $900 quote is brutal and I'd be livid too. I got burned on a used prep table years ago, seller had it running when I showed up but it was cold because they'd iced it down right before I got there, learned that trick the hard way. Now I always bring a thermometer and make them let it sit plugged in for at least 20 or 30 minutes while I stand there watching it cycle on and off. I also pop the panel and look for rust, oil streaks, or any signs someone's been messing with the lines, and I ask to hear it kick on from a dead cold start. If the guy won't let me do that, I walk, no matter how good the price looks.