1. Can I declare the money as “Star-up Cash Flow”? Do I have to paid taxes again on the start-up money?
You don't pay taxes on the investment of capital, you will only pay taxes on any profits made by the business, interest, dividends
or capital gains. As you said, since the principal has already been taxed, it would only be increased wealth from it's investment that would create another taxable event.
2. Or this money would be seen as owner equity in the business?
That's right it's owners equity and would coincide with any assets that it's used for on the other side of the balance sheet (Assets = O.E. + liabilities)
3. Do I have to explain the IRS why I keep that much cash money under my mattress?
No, the IRS is only interested in INCREASED wealth, like gains from it, interest on it, dividends generated by it. ... If it's in your mattress, then there's been no increased wealth from it.
...hope this helps! and good luck on your venture.
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