I'm with evand; if you pay by cash and don't get an invoice (when you normally would want or expect one), you
know what's going on and you are a party to fraud (ethically, even though it may be hard to prosecute).
Of course, keithj, bargaining at electrical discounters is not in this category - you get an invoice and the transaction is very obviously recorded.
josko, I'm delighted to hear that there are tradies around with such high ethical standards as your friend. I bet it was a good deal, anyway.
Sue, your position sounds suspiciously like "I don't care if they rip off every taxpayer in the country, as long as they don't rip off ME"!
Which from a practical perspective is probably a position that nearly everybody would take, but it's pretty hard to defend as the most ethical position.
I recognise that life is full of contradictions. What's ethical and what's practical are very often two completely different things. Where people get into murky water, IMHO, is when they don't recognise these contradictions, and attempt to justify their pragmatic position on ethical grounds.