Innocent until proven guilty" is an idea rooted in Jewish ethos which teaches that all have a 'presumption of honesty'. Yet at the same time our Sages teach us that most sin in the area of monetary acquisition. The key is not providing tempting opportunities for financial improprieties; doing so renders one an accessory to the act and causes one to be in violation of the Biblical interdict of placing a "stumbling block before the blind". An auditor who ignores "red flags" in the clients accounting records and meekly signs on the dotted line is at least as culpable as the actual perpetrator of the wrongdoings.
Being party to a transaction where income is undeclared would be prohibited. One of the basic rights of governments is that of taxation and Judaism sees the payment of such taxes as a 'religious obligation'. This obligation was formulated at a time when taxes collected were used to enrich the royal treasury at the expense of the populace. All the more so in a democratic country like Canada where taxes are (for the most part) used to benefit society as a whole, and even our "kings" must pay taxes. Those who illegally avoid taxes are not just cheating the government - but are actually stealing from their fellow citizens; those who habitually evade taxes are deemed unworthy of testifying in a Jewish court of law - their word can not be trusted. The fact that governments waste some of our tax dollars is no excuse for shirking our legal and moral obligations.
Thus when one knows or strongly suspects that taxes will be avoided then a purchase should be made elsewhere. Such cases would include being told so directly by the vendor, the demand to make cheques payable to cash, the refusal of a retailer to use a cash register or to issue a receipt. One should not assume, however, that all who request cash are doing so dishonestly. Often the convenience of cash makes its use desirable. After all it is legal tender. Examples might include a garage sale or even a "home business" where receipts and cash registers are used.