Is buying a term paper illegal in any state or under federal law?

Unethical, yes. But illegal?
I don't think so. As you said, definately unethical. I suppose, depending on the circumstances, it could be considered fraud.
no
If you summit it to a school as your own, it could be fraud, ( doubt if anyone has ever been charged) but very possible that you would fail that class or even get suspended from school
im not sure, but i am pretty sure you can get kicked out of college for it so just weigh your options. is it worth it to you to get kicked out and get a black mark if you ever try to apply to any college again.
It violates most school dishonesty/integrity policy. Check the school websites and usually it's clearly written there: what is considered cheating, and the consequences.
It's called plagiarism, and yes, it's illegal in all 50 states. Write your own, or at least use parts of one that you buy and put it in your 'Works Cited' page. That way, you're covering your butt.
Most colleges now have software that can take a questionable phrase, search internet databases and websites and it WILL prove whether or not you wrote that paper.


Answers:
No it is not illegal.

But be warned, doing so puts your education in Jeopardy. If you get caught, you will face up to explusion. Meaning that no college will ever admit you since your college transcript will note that you have 'academic dishonesty by plagerism.'

Also noted, experienced professors can read essays and notice patterns of plagerism and if they know your writing style well, they will notice that something seems different.

Colleges even have essay submission programs that will check if everything is legit or not.