Depending on the circumstances, yes, it very well could be. At the very least you should have contacted the sender and notified them of the error so that they could rectify the error or reclaim them if they wished. If you do not, and "convert them" to your own use you have committed "taking property without right."
Illegal
No. But should you choose to resell them once they have arrived, you will run into state and federal laws which address selling or trading counterfeit goods.
Illegally importing or exporting goods is called smuggling
Illegal importation of goods is commonly referred to as smuggling.
Black Market
sure it is!
Yes. By definition, black market activities are those that are dealing in illegal goods.
I'm afraid yes, but not seriously illegal.
Items such as food or materials that are not illegal to trade.
smuggled
Nope - at least not in the UK. Here, it's illegal to sell goods that are out of date.
Yes, it is illegal to pre-post mail. It is also illegal to pre-date a check that you are giving someone to pay for goods.