It's a cash business so the financials are only going to show break even numbers or a lose.
This says one of two things:
1) It doesn't actually make money, or
2) The guy is lying about his income to the federal government.
If he is a liar and willing to break the law to do so, why would you think he isn't going to lie to you?
I don't deal with people like that. When I come across a real estate opportunity and examine the person's tax returns and they've claimed way more in expenses than is being represented in the transaction (i.e. they're trying to show higher profits in their sales pitch, but the numbers don't line up with their official reported numbers), why would I deal with them? I can't trust them to be straight with me.
Get the financials. That's the true measure of both the character of the seller and of the profitability of the business.