If you receive a fake product from a supplier, fast action is essential. If you are indeed working with a fraudulent seller, they could disappear quickly and there will likely be a contractual time limit to getting a refund.
Gather proof that the items are counterfeits, present these to the seller and ask for an explanation. If they offer a refund, make sure they absorb the cost return shipping - the threat of further action can help you be more convincing, but remember to always stay civil and calm, as it's possible the vendor was unaware of the problem.
If you've paid for a shipment with your credit card or PayPal, you will probably be protected against fraudulent dealings. Each company will have different procedures for processing disputes, so get in touch with them to find out how to proceed.
Consider leaving feedback about the situation to warn future customers about potential problems, but be sure any claims you make can be supported, as you don't want to be caught with a libel claim against you.
This will vary, depending on your location, the location of the seller and the specifics of the situation, but in most cases you should get in touch with the authorities to report the goods.