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".
This is only if you unknowingly purchased stolen goods.