I admire the zeal of many of the posters here, if only everyone had such moral fortitude we'd need less revenue auditors..!
I should point out though, that the statutory test for a deduction for computing the profits of a trade for tax purposes, is that the amount was laid out wholly and exclusively for the purpose of the trade.
The standard of proof required is in effect the civil burden of "balance of probabilities".
So - unfortunately, or equitably, depending on your personal stance - if someone is caught for having operated a trade within a trade, they will be allowed deduction for expenses paid out of the undeclared receipts, be they wages, materials, stock or whatever.