As the customer "morally" we don't know whether the tradesperson declares it, for all we know he does. Ok there is an assumption that cash in hand transactions are not always declared but we cant tarnish all builders, window cleaners, barbers, ice cream man etc......... with the tax avoidance brush. As a youth I did some labouring work, the guy paid me £200 per week in cash, never told me anything about tax or self assessment forms etc.... It was only for my parents telling me I needed to declare and showing me what to do. I could have run that risk, I was honest from the start