A few weeks ago, there was a discussion about shaders, textures, and reliability on vendors to use them properly.  Suffice it to say, the thread has since made me wary of buying user-created content from this site or anywhere else.  In fact, I've had first-hand experience with just how poorly done they can be, as explained by another artist while working with a dress I once bought.  Unfortunately, I'm still very new to learning how materials and shaders work, and don't yet know enough to identify or fix such problems.

Is resolving those issues just a matter of learning how to use the material room and understanding how shaders and textures work?

Please disregard this post.  Reading it over again, I realize I answered my own question.

In a word: Yes.

If you're really interested in how to, um, correct most vendors' attempts to create an effective node set-up, then there are a few things you can do:

• Read around on other CG discussion boards, especially the more "pro" ones. Yes, there's a whole bunch of stuff that will be beyond your experience AND will not apply to Poser, but you'll also become familiar with the bits that are. And maybe pick up some things that, later, you'll go "Aha! I remember reading about such-and-such!" and you'll have a better idea where to start investigating a particular feature.
• Don't be afraid of learning how to implement some basic arithmetic using Poser nodes. It's incredibly simple once you work out what's going on. Many proper materials require that one value be the inverse of the other. Learn how to do this using Poser nodes (well... just one for a straightforward inverse function) so that you can do it in your sleep.
• Learn some elementary physics about how light interacts with surfaces. Stuff such as how reflections are tinted depending on the property of the material they're bouncing off. (This will allow you to correct a large amount of shaders where the vendor/whoever has plugged a diffuse colour map into the specularity colour... some materials require it, other materials need only white in the spec. colour, for example. Knowing which is which will help your materials look better.)
• Read the materials threads here and in the Node Cult on the RDNA forum. Especially those posts by Bagginsbill where he explains in detail what one of his materials is doing and why (the "why" can then be taken and used for other materials you want to try).
• Don't be afraid to try things and then ask questions about the results (here or at RDNA) if they're not as you predicted they'd turn out.

EDIT: Please don't disregard my reply, though! I typed it up and pressed "Post" and everything!

Adding to Johnpf's fine note: if you happen to see disparity between what's written in the Poser manual in the material room section and what Bagginsbill has to say on the subject, distrust the manual and stick with Bagginsbill.
A lot of the erroneous shaders that you will find in the marketplace come from trusting developers following the manual.

Another thing that the recent efforts to export materials and Poser objects to LuxRender has revealed: a lot of what we do in terms of trying to achieve "realism" or physical accuracy or whatever you want to call making-an-object-look-like-what-it-is-supposed-to-represent is a kludge... an artifice. A cheat. A workaround. So, shaders developed for Poser/Firefly don't really work too well for other renderers.

Just to add, many earlier content were created in previous Poser version which exhibit inherent built-in problems of their own.  These were correction factors because there were simply no other way to get around the issues.  One of the most common, active ambient channels for human skin.  Turn off all the lights and you have a model glowing in the dark.  And why was this done, to offset the lack of understanding about gamma correction and how it applies to linear rendering.