Let's say I'm a customer. There is a seller who is offering me a deal in such a way that it is obvious he's trying to avoid paying taxes. (For example, the seller insists on cash payments and a lack of documentation). Do I have a halachic obligation to insist on a more trackable form of payment that will be harder for the seller to hide from the tax authorities?
Just to save some speculation, in both the US and Israel the obligation to pay sales tax/VAT falls on the seller, not the buyer, and the seller is required to report cash transactions (although obviously not all do). So as far as the state is concerned I am blameless if the seller decides to commit tax fraud. My question is if halacha says something different.