If a site does not explicitly say whether or not you have PLR/Resell rights, and there is no copyright anywhere on the site, does this mean you can resell anything bought/downloaded there? If you have a product that you sell on your site, do you have to explicitly say that it is copyrighted or that there is no resell on this product?
Everything has an automatic copyright - no notice is needed. If you do not have express rights to use and/or permission to resell, then you cannot - so if it does not say you have resell rights, then you don't.
Correct. The default is "copyrighted". Recent works are only in the public domain if there is a specific notice stating they are in the public domain. However, there are enough people out there who are ignorant of copyright laws that it can be useful to say "copyright" on your site somewhere so that there is no confusion.