Greetings, I’m new to Switch repair and electronics repair in general, although I do know how to solder. I was wondering what are some things to watch out for when buying broken Switch consoles. Are there any symptoms that could be indicative of a more serious problem? The first console I purchased only had an issue with the left joycon, but it turned out that the previous owner had botched a hardware mod lol.
A common thing to look for especially on auction sites it stripped or missing screws, a crack by the charging port and most importantly look at what else they sell and look at feedback, I found a few as “not tested” and they have clearly sold loads of switches so a false claim straight away
In my humble experience so far, a lot of them are fixable even if they appear to be in poor condition at first sight. If you know how to combine hardware and software knowledge (a lot of emmc system files defect are fixable) you should not be afraid by anything. Just keep patience and faith 
Just, do not be stubborn when you are stuck. Switch on another one and come back when you have accumulated more knowledge to fix what seemed to be non fixable.