We need to distinguish between counterfeit items and “knockoff” items. Counterfeit items use the trademark of an actual company/product and pass themselves off as the real thing. Knockoffs do not use the trademarks of others and attempt to only look similar. Counterfeit goods are illegal to sell, knockoffs are legal. They also have a moral difference:
Counterfeit items steal the intellectual property of others. To knowingly purchase such items is to contribute to theft. The purchase of counterfeit items affects the intellectual property rights of the inventor/designer and also the jobs of those who labor to make legitimate goods as well as those who own and are employed in the stores that sell them. On top of that, counterfeit items are often sold by criminal organizations so purchasing them contributes to crime, violence, and forms of exploitation. So, yes, it would appear to me that buying counterfeit items is an unacceptable form of cooperation with evil and therefore sinful.
As to whether or not you can keep them, I don’t see a huge problem with that. The damage was done when the purchase was made, no further harm is inflicted by keeping the item(s). However if you have children you might want to think about what kind of example you’re setting for them.
