If state actors use the 5$ wrench, that's illegal and consequently you have the defense of the courts to counter that behavior. Civil forfeiture is dependent on being able to "prosecute" your stuff, rather than you. So if they can't seize it, they have to go after you directly, which puts the burden of proof on the prosecution to demonstrate that the money is unlawfully possessed. The fundamental principle of the US court system is "innocent until proven guilty", so you don't have to prove what your income is, they have to prove it. You only need to counter their claims.