It's not really illegal in the "go to jail sense." But you can definitely be liable for patent infringement if you give it away, even if they aren't selling it.
That said, there is no reason to assume a product is covered by a patent unless it's marked with the specific patent number (a patent marking on it's packaging counts too). It might be, but most products aren't.
If you are an unknowing infringer (didn't know and had no reason to believe you infringed), your damages are limited (in the USA) to a "reasonable royalty." Basically, what a the parties would have hypothetically agreed to in a negotiation. That might be a few % of revenue or a small lump sum.
That said, there is no reason to assume a product is covered by a patent unless it's marked with the specific patent number (a patent marking on it's packaging counts too). It might be, but most products aren't.
If you are an unknowing infringer (didn't know and had no reason to believe you infringed), your damages are limited (in the USA) to a "reasonable royalty." Basically, what a the parties would have hypothetically agreed to in a negotiation. That might be a few % of revenue or a small lump sum.