Is it illegal to make a solutions manual for a textbook and sell it online?

Answer 1

VERY likely! Publishers and authors strongly guard their copyrights and any amplification on their original works may well fall under the copyright law.

But from a quick search in the internet you can find many website selling thins kind of products.

Answer 2

Yes, selling a solutions manual for a textbook can be considered copyright infringement and is illegal without permission from the copyright holder. It's important to respect intellectual property rights and not profit from someone else's work without permission.