Google Books is a service voluntarily provided by Google. They have no obligation whatsoever to make any particular book available to any extent, or even to keep running it. Also, no one is forced to use Flickr. There are other photo-sharing sites, and if you really want control you can buy a domain and make your own site.
As for selling public-domain work, let's look at some definitions:
Public domain: "not subject to copyright"
Copyright: "the exclusive legal right, given to an originator or an assignee to print, publish, perform, film, or record literary, artistic, or musical material, and to authorize others to do the same."
There's nothing about having to make it available freely, and people sell copies of public-domain works all the time (e.g., classic books, Shakespeare, hardcopy of the US Code). At the same time, you still can't claim it as your own work - plagiarism still applies, as Fibanocci8 pointed out.
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