I suspect that Microsoft required a payment to test products for compatibility. You're sort of getting your suggestion through stores that do a quick check that content works correctly. The best developers go through a quick playtesting process by letting a few people try out their product before marketing them. Don't forget that DAZ studio also has a large following and can have a different installation path.

Most commercial products meet your requirements. PNGs are in all newer products because any item saved to a libary (Poser 5 or later) generates a PNG thumbnail. Developers had to do extra work to save an RSR thumb to maintain compatibility with Poser 4. You're stuck with older products that only provided RSRs unless a store wants to repackage everything into new installers (not likely to happen). Commercial items provide readmes that typically will state at a minimum the store license and locations of files (though may not provide much info on how to use the product).

Freebies are a mixed bag for the simple reason that people are donating something for nothing. For the most part, even freebies do a decent job of providing a good installation. When they don't, it's hard to complain (though I've seen posts by freebie providers that are sick and tired of the number of complaints they do sometimes get).

What you'll probably find as you install more and more content into Poser is that you've got to rearrange everything anyway even if installations are good. You'll want to group like items together where they usually get installed in a seller's named directory. Unless you decide, for instance, to group all swords together, you'll have to otherwise try to remember what products have swords and search all over your runtime to find them. I've tried to brainstorm a logical way to install everything, but there simply isn't one that will be satisfactory to everybody. If you were to start a thread to see how everybody organizes their runtimes, you'll probably find a hundred different ways of doing it.

If you get a bad installation from a store, contact the store to see if they'll fix it and provide a new download. If it's a freebie, just thank the freebie provider for giving something away. If a freebie provider gets more complaints than thanks, they'll think twice of offering one in the future.

My visual indexes of Poser content are at http://www.sharecg.com/pf/rgagnon