If you pass off someone else's work as your own, you are in breach of the law. If the artist who created the original happened to spot what you did with his/her work, you could be taken to court for infringement of intellectual property.If charities recycle Christmas cards for good causes, that is probably not going to cause trouble, but if you make a profit for yourself, the original artist could claim that it should have been his/her profit, not yours.
You can be inspired by an idea, but you may not use someone else's work at least until they have been very long dead.