/ic/ - Artwork/Critique

Is it illegal for me to purchase a painting, and then retouch parts of it, or change parts of it, and resell it at a higher price?

I purchased a painting that was by an unknown artist, the person selling it wasn't the original artist, and I want to resell it for a profit, but the face is atrocious and there's some damage and chips to the paint.

What do you guys think? Reddit threw a hissy fit and started yelling about copyrights and me having no morals.

You are given the opportunity to go back in time to your past, still novice (or more novice) self and give yourself two art related pieces of advice: On thing you suggest you should do, and one thing you suggest you shouldn't.

What do you say?