Cashing checks is not illegal. Failing to report your income is illegal. Although, they have to be able to prove you evaded the tax.
All banks are required to report to the FBI if they feel you are doing anything suspicious. They are also required to report if you hold more than 10000 in an account at any one time. There are many things that banks are required to report, the bottom line is, you need to pay taxes on income, and keep detailed records of income and expenses.
If you try to go under the radar, you'll be sleeping under a guy nicknamed radar in prison.
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