What happens if you declare it? Could they confiscate it without due process, like the police can in many states?

If you don't declare it, it's guaranteed to be confiscated until you prove a legitimate purpose for the > $10,000.

Federal authorities generally have a higher standard of behavior than local or state police, although obviously not in every instance. If you try to cross the border with $500,000 declared, they may have some questions for you.

I was under the impression if you don’t declare it then there is no legal burden on the government to return it to you, whether you have a reasonable explanation or not. Whereas if you declare it and have a reasonable story/sufficient paperwork they can’t/shouldn’t take it.

The point being that in cases where there is no funny business (at least in your opinion), you are strictly better off declaring it than taking your chances on not doing so.

I feel a lot more comfortable wiring the money or using checks or so on than traveling across a border with that kind of cash.

>Whereas if you declare it and have a reasonable story/sufficient paperwork they can’t/shouldn’t take it.

The law in the US is that, the police can take anything they want from you, with zero judicial process needed, slap it into their department's slush fund, buy whatever toys they want, or even as someone posted above, MAKE IT PART OF THEIR YEARLY BUDGET PROCESS TO "EXPECT" MILLIONS IN STOLEN MONEY, and unless you can AFFIRMATIVELY PROVE in a court of law that you obtained the property through zero criminal or near criminal activity, you will never get it back