Selling something you purchased vs. selling something you got for free, with the express understanding that it would not be sold are very different situations. Extreme or not it is clearly stated in the rules to use the forum. I would view the examples you've given as recasting
ANY selling of a prop, whether it is a pep file base helmet, or a book cover used on an existing book should have the express permission of the paper prop creator (artist) to sell (if the file was in the free prop section), and I assume offering some form of financial compensation to the artist (since we're talking about selling it for money).
Example: I modified a kindle case to resemble the Hitchhikers Guide to the Galaxy as a gift last Christmas, the artwork was available for free in the paper prop section, so if I print the art for the front/back covers and the spine of the book would it be OK for me to make just one and sell it?
The answer is NO.
If I wanted to do that I would contact the member that posted the files and see if they were interested in a business transaction, so they could be fairly compensated for the time they spent creating the files.
The reason I (or anyone else) use the free paper prop section is because I don't have the skill set or talent to create (or replicate) most of that artwork, the people that do have the talent have VERY generously offered to share their talent with the community so that we can all enjoy this hobby more.