It is if you happen to buy stolen property. You might not be held responsible for stealing, but you'll have to return the property and say good buy to the money, I think.
IANAL, but... They owe you and it is a question if they can get it back to you, but the rightful owner has precedence in many (most?) cases. That's the setup for many kinds of scam. The scammed party buys something, but the transaction of the property is in some way faulty, while the money quickly disappears.
The law is also going to ask hard questions about "unknowingly". If you're walking around Market St. at 11 PM and some dude stops you and tries to sell you a fancy new bike for $200, complete with a U-lock holder but no U-lock, the law is probably going to say that you should have expected it to be stolen property. If you gave the guy $200 in cash without asking any questions, you probably won't get away saying "I unknowingly purchased stolen goods".
Sure it is; that's why it's called "due diligence". You are legally required to engage in some amount of diligence when acquiring a company. As a very related example, a US company acquiring a foreign company is obligated to make sure the foreign company isn't engaged in corruption/bribery, and is expected to show a certain amount of competence in investigating the foreign company before the acquisition. Going ahead with the acquisition after an incompetent examination is illegal.
More generally, the law recognizes the concepts of negligence, reckless endangerment, a "reasonable person," the idea that you're responsible if you "knew or should have known" about some problem, etc., all of which I would classify as specific ways of stupidity and incompetence being illegal. Of course if you keep your stupidity and incompetence entirely to yourself, you're fine. But if you act stupid or incompetent in a way that affects other people or the public, chances are high that you're in legal trouble.