When is something considered to be in the "Public Domain"? If there is a website with no copyright markings is it in the "public domain"? Can I use/sell anything in the public domain without permission?
No, not having a copyright does not mean it is in the public domain. Anytime you write something original, take a photograph, or create a piece of art, it has an automatic copyright. http://en.wikipedia.org/wiki/Public_domain
Oh! Gamerfreak don't do that. Those are not public domain materials. If you need to get public domain stuff, (pictures, books, articles, sound, etc) you can start looking here: http://en.wikipedia.org/wiki/Wikipedia:Public_domain_resources
If something is clearly listed as "public domain" then you are in the clear. Everything else likely belongs to someone, somewhere.
If there is a car with no label "this car belongs to xyz", can you then steal it? If there is a purse on the ground with no label "this purse belongs to xyz", can you then steal it? Why does everyone think the internet is different?