Copyrights have expiration dates so to speak. When a copyright has expired they become what is known as "public domain". This is also why Disney fought hard about this in regards to Mickey Mouse. Once it reaches the "public domain" you may use it even if you can make money off of it without paying royalties.
For Example:
Works first published before January 1, 1923 with proper copyright notice entered the public domain no later than 75 years from the date copyright was first secured. Hence, all works whose copyrights were secured before 1923 are now in the public domain, regardless of where they were published.
Works published and copyrighted 1923-1977 retain copyright for 95 years. No such works will enter the public domain until 2019 unless one of the other rules applies.
Works first created on or after January 1, 1978 enter the public domain 70 years after the death of the author if the author is a natural person. (Nothing will enter the public domain under this rule until at least January 1, 2049.)
Works first created on or after January 1, 1978 which are created by a corporate author enter the public domain 95 years after publication or 120 years after creation whichever occurs first. (Nothing will enter the public domain under this rule until at least January 1, 2074.)
When an item is copywritten, the copyright must be renewed before its 27 or 28th year for an extension. (Once again back to Disney as they had to go to court over the mouse).
So In reality, You could use "public domain" works to sell without having to pay royalties, but you can not for example say sell "Crime and Punishment" and slap your name on it as the author. But you could offer it on Ebay for a fee as say an ebook for download. If the person wants to pay you for that instead of using their head and downloading it free online then oh well.
If you want to read more on copyrights or get some great books to read go here:
Project Gutenberg. In case you did not know Gutenberg was the man who is credited for inventing
moveable type printing (printing press) and also known for the
Gutenberg Bible.