Markphd, what exactly are you talking about? What are you referring to as illegal? A buyer asking a seller to insure the item for less? A seller insuring the item for less? Or both?
Like I said, there should be a difference between declared value and insured value, but I don't believe anyone including the shipping companies do much to separate the two, so the insured value ends up reflecting declared value. The shipping companies will tell you that it's illegal to insure it for more than what your product is worth because you might end up cheating the shipping company, but it's not illegal to not buy enough insurance. If I'm shipping an item that is worth $5000 to me and my buyer, I can see how it's illegal for me to declare it to be anything less, but it doesn't make sense for me to go to jail because I refuse to pay $5000 worth of insurance or for my buyer to go to jail because he refuses to pay. That cannot possibly be a crime (again the bigger problem is that the insured value is treated as the declared value). In Canada, it is illegal to drive a car that does not have insurance, but we are not required to purchase extra insurance just like we are not required to purchase extra home insurance.
And lets not forget that there's no law or official pricebook governing the price structure of used audio gear. A product can be priced at whatever amount the seller whiches to sell it at. Perhaps you're not telling us something about your friend and what he did that may help explain why the police went to his place. At the very least, since it's HIS products that got confiscated, he's not the seller, and as it should be, if it's the buyer who requests the seller to insure the item for less, the seller should not be responsible for anything other than the insured value including customs and duties. I think almost everyone here would agree with me that this is the accepted norm.