Izzo......you are correct that you can modify some creative commons pieces but you are not allowed to pass it off as your own or resell it. That's why it costs so much more money on stock sites to use downloaded art or photos on t-shirts or even screensavers. So unless you plan on using these kinds of 'cc' items on ads or collateral in general, it's illegal.
Here's another example: You've decided to create a logo and have simply modified an existing vector art and sold it as an original logo. The client cannot legally copyright or trademark their brand. Another scenario is like the postcards. If you wanted to copy an existing postcard that's even 80 years old, the company that originally produced them for sale has to have not renewed the copyrights and aren't still making $ off the art themselves. Disney is a fantastic example of that. Their copyrights/trademarks are still active.
So once again, it pays off to be original.