Would consumer protection laws help?
Charging for making copies of public domain works available seems fine to me. There's obviously work involved in producing copies for people to use.
But you shouldn't be able to claim any copyright on the copy. So if I buy one, I can then copy it myself without restriction. So the most you can realistically charge for the copy is whatever it'll cost the average person to make their own copy.
Asserting that you own a copyright on such a work is indeed fraud if you attempt to extract a greater price from me based on that false assertion.
It might be interesting (in the UK) to take such cases to the small claims court under consumer protection law - for example if you bought a book where the publisher claimed copyright that they did not own and therefore knowingly mis-represented the goods.
The small claims court is a good way of tipping the balance of legal costs in the consumer's favour so it might be worth a try.