Ex libris is Latin for "from the library of..." It would traditionally be followed by the name of the person who owns the book: Ex libris Tem42.
When selling used books the description ex libris (or ex lib, or ex-library) means that it is from a public library. It may have the library's stamps, card-pocket, or call numbers on the spine. It will otherwise be normal, unless it is stated otherwise in the description. Library binding, lack of dust jacket or plastic covered dust jackets should be stated in the description.
Usually a book will not be knocked down from VeryGood standing just because it is ex libris, but it should not be labeled as Fine or Like New. If you are buying a book for your personal enjoyment and don't care how the book looks, there is nothing wrong with ex libris books. Library binding is usually a good thing, meaning that the book will last longer.