You seem to have a good knowledge of several terms that relate to law, but unfortunately the underlying concepts elude you. There is a reason that individuals usually go to law school for three years, and have to pass a rigorous examination before being able to practice law.
First, the Commerce Clause is not a basis for taxation, that would be Article I, Section 8: "The Congress shall have power to lay and collect taxes, duties, imposts and excises..."
Second, the First Sale Doctrine is not a constitutional doctrine, but a principle from the Copyright Act that an author loses the ability to control the distribution of a copyrighted work after it is lawfully made and sold. 17 U.S.C. Sec. 109. It does not relate to taxes whatsoever.
Third, it does not matter whether the federal government has authority under one part of the constitution for its regulation if it also violates another part of the constitution. For instance, a regulation that is valid pursuant to the Commerce Clause but which also violates the First Amendment is nevertheless invalid. See, e.g., Reno v. American Civil Liberties Union, 521 U.S. 844 (1997). Therefore even a valid tax under Art. I Sec. 8 would be invalid if its violative of the First Amendment's Free Speech Clause.
Fourth, while your description about direct and indirect taxes is mostly correct (though not actually related to the question posited), your example about what may be income, "(loans, etc.)," is wrong. A loan is not income because it comes with an offsetting obligation. See Commissioner v. Glenshaw Glass Co., 348 U.S. 426 (1955) (giving the three-prong standard for what is "income" for tax purposes: (1) accession to wealth, (2) clearly realized, (3) over which the taxpayer has complete dominion). The offsetting obligation to repay the loan means its not an accession to wealth.
Finally, your statement that a state can do whatever it wants as long as it doesn't violate its own constitution is clearly erroneous. State action as well as legislation are invalid or illegal if it violates the Federal Constitution. Often if a state actor violates the constitution you can sue for money damages under 42 U.S.C. Sec. 1983. Examples of state legislation being invalid would be, for instance, if a state and federal regulation were inconsistent. The state regulation would be preempted under the Supremacy Clause. See, e.g., Edgar v. Mite Corporation, 457 U.S. 624 (1982). Also, a state law that would violate the federal constitution is just as invalid as if the federal government had passed it. See, e.g., Lawrence v. Texas, 539 U.S. 558 (2003) (striking down Texas sodomy law as violative of the Due Process Clause of the Fourteenth Amendment). An even better example is Romer v. Evans, 517 U.S. 620 (1996) in which the Supreme Court struck down part of the Constitution of the State of Colorado as violating the Due Process Clause of the Fourteenth Amendment. If part of a state constitution can be unconstitutional under the federal Constitution, a fortiori any state law passed pursuant thereto could also be held unconstitutional.
While I personally applaud any attempt to educate people about the law, this is not the way to do it. Giving people misleading or just plain wrong information can make them worse off than if they never had the information in the first place.