What do I do if I read a short story on a writer's website and I really like their idea, and I want to take it and expand it into a book? It's not my idea and I would feel bad about taking someone else's.
This is not common practice for actual published authors. Published authors are faced with tons of copyright/plagerism claims, and if they are caught have to pay out thousands. Famous authors in America literally have to throw out tons of manuscripts sent to them because if they accidentally write something similar, a lawyer will anally rape them. Very common in Hollywood/scriptwriting world too.
If you are unpublished and never going to make it, fine, do it. If you actually are a professional, don't even get near it.