For example, if an Amazon competitor had the exact same product categories, or if a Craigslist competitor had the exact same categories...I'm mostly talking about how you categorize what you're selling.
Is that "trademarkable"? Can you be accused of infringing by using someone else's scheme?
The copyright protection concerns only "original" creations. A list of categories or products is not an original creation.
As far as it is a "logical" classifying that follows some comment sense, it can't be trademarked and it can be reproduced.
(if it is really original classifying or if it derives completely from the common sense, than it can be considered as "original creation" and thus protected by copyright - but that is IMHO a very rare/unique case - certainly not the Amazon product categories listing)
More about the copyright's scope: http://en.wikipedia.org/wiki/Copyright#Scope