I would say that it's best to try to understand their insider knoledge of the industry but also WHY people buy from them.

What do you mean by insider knowledge?

Yes, things that real customers actually want to hear, how competitors really stack up, etc.

Would it be okay if I PM you some questions about sales?

Just post them here and people will answer.

Company has horrible reviews from customers, and I am thinking that is a red flag. Should I still even go to the job shadow?

How well they know their industry/other top players in it/the product they're selling

What are things you wish you knew your first day? What have you found is most common among top performers that contribute to their success?

Definitely get a nice cold beer