Perry Mason Fans: Seeking Info on Used Car Dealerships

I am taking a class in Litigation Support as part of a certificate in Forensic Accounting (Northeastern University CPS ACC 6240). This term we are building a case for a client that has to do with income derived from a dealer of used cars. There is a lot of info that I need to gather this first week, and I am hopeful that folks here will help me out. ALSO, to substantiate that any info you give me is accurate, it will be helpful to know your credentials – for example you have owned a used car dealership in MA (the state in question for this case) for 20 years, so that my expert witness credibility will not be destroyed on cross examination!

How do you determine what value is placed on a car when you (the dealer) buy it and when you sell it? I use KBB, but do they have a wholesale price and a retail price? How do I find these? Are there other authorities for this kind of information?

Are there particular taxes that car dealerships have to pay?

Is there particular regulatory reporting on cars that are sold (make, model, year, price, etc.) that needs to be reported to some entity? Does anyone know what that entity might be in Massachusetts?

Are there other screwy things that are unique or unusual for MA car dealerships that I, as a pretend expert witness ought to be aware of?