First, I'm not a lawyer, but I have been a writer for some time and understand copyright to a degree. It's best to do your own research on the particular area you're looking at. but here's my two cents as I understand it.
As you suggested above short things people have said in public are usually ok, unless it is a trademarked catchphrase.
Trademarks are phrases associated with a business and are much harder and more costly to secure. They're not usually granted for common phrases or ideas, and therefore strongly pursued if someone uses them. They are also often limited to a certain business or product.
For instance, you may be able to write a book titled, "I'm lovin it," that had nothing to do with McDonald's but you couldn't use it in your restaurant across the street.
Copyright for books, and movies refers to the entire work. That is why book and movies can have the same title and be different. The title is only copyrighted as a part of the larger work. Most of the time it is okay to use a short quote...longer than a sentence or two requires permissions.
Now here is where it can get tricky. Because of the decline in book sales some publisher have started selling products with phrases from books as gifts. They secure additional rights to do so, such as Max Lucado's publisher.
Pulling a quote from a movie may be ok, but I wouldn't use a star image or pictures from the movie. Movie posters, images, and the star's images are often heavily trademarked. Make my day on a t-shirt because it has become a common phrase popularized by the Dirty Harry movies is probably ok. Adding Clint Eastwood or anything else associated with the Movies wouldn't be. For a time after a movie has been out key phrases may have been trademarked because the franchise is going sell products based on the movie.
For books you can checkout the Gutenberg project which is the oldest collection of digitized books that are in the public domain. They encourage their free reproduction and distribution.
Lawyers will always argue over what is and isn't ok. There are no additional rights as as Etsy seller, like all businesses you have to do your best to operate within the law.
I hope that helps.
