I think you are mistaken in your beliefs about the nature of the marijuana industry in Oregon, but setting that aside...

"It’s inconceivable to believe total honesty will prevail..."

This I agree with. Not because of "dopers", but because of human beings. It's inconceivable to believe that total honesty will prevail with any group of people who are required to pay taxes. That's why there are rather strict accounting and reporting requirements, as well as tax fraud investigators for all businesses. The tax people don't assume that anyone is honest.

And selling pot outside of the system, old-school, is just as illegal as it has always been -- in addition to involving tax fraud.