Here is a great blog post that provides a bit more detail: The Importance of Foreign Rights. This is an incremental revenue stream that many Internet marketers/authors may not have considered before now. And it's an important one because there are literally thousands (possible even millions) of additional dollars to be made from a single book if marketed properly to the worldwide publishing market.
Where can authors buy and sell their rights? It's probably best to find a foreign rights agent to help you with the first one or two projects because they not only have relationships with all the "movers and shakers" in publishing around the world, but they know how to word the contracts for best results. You can find agents, publishers, and basically everything you're looking for through PubMatch.
Check it out. It's yet another potential revenue stream for your online business.