An interesting argument, Umbran. Here, though, is the reason I gave the theory that I did.
The purpose of existence for a role-playing publication is to aid role-players in enjoying themselves.
In order for a role-playing publication to aid role-players in enjoying themselves, it must be used by role-players; if it is not, it cannot be the cause of their enjoyment.
A "good" thing of any stripe is that which most accomplishes its purpose for existence.
I think the quibble here comes in defining "most accomplishes." Some people say that means that is does an excellent job on the "micro-scale" - i.e., it is an excellent product because it helps my group enjoy itself due to (a) good writing, (b) good rules, (c) good flavor text, (d) good art, (e) good other, (f) all of the above. This is what I think a lot of people want to mean when they say something is "quality" - it does a "good job" describing a game (or a setting, or whatever) that the person saying it is "quality" would like to play in - whether that is for setting, rules, elegance, simplicity, and so on. This is a valid definition of "most accomplishes" but is horribly subjective because it is almost impossible to get any two people to agree upon exactly what set of rules is best. (Hence, the great abundance of house rules, not to mention different RPG systems). Other people say that "most accomplishes" should be looked at on a "macro-scale" - i.e., it is an excellent product because lots of people are using it (even if my particular group isn't) and thus the total enjoyment it is responsible for is high. The problem, of course, is that role-playing is a "personal" endeavor, and most people like to think of their own way of gaming as The One True Way, and anyone using any other rules isn't having "as much" enjoyment as they might have if they gamed using the One True Way. Of course, he's over there looking at you thinking you'd have much more fun if you'd subscribe to his One True Way.
At the end of the day, even sales aren't the best indicator of what is actually being played - I've bought a lot of things I've never played. But it seems to be true that in order for a product to be played, and thus aid in providing enjoyment to gamers, on either a micro- or macro-scale, and thus serve its purpose for existence, and thus be a "good" product,
it has to be bought first. And that's the bottom line.
"Micro-scale" judging of quality is in many ways useless because, as I mentioned, no two people think exactly alike. What I think is "the bomb" might be the exact opposite of what you like. Thus, we're left looking at "macro-scale" judging - i.e., not "what do I play" but "what are most people playing?" Sales do not perfectly reflect play, but a low-selling product can't have a high number of people playing it.
Again, my theory is simply:
High product sales indicate a high quality product. High sales imply high rate of play. High rate of play implies high enjoyment among RPGers derived from the product. By my own definition, that is (macro-scale) quality, QED.
It's not perfect, because what I really want to say is "high rate of play indicates a high quality product." But since it's almost impossible to quantify rate of play and since rate of play is a function of rate of sales, I go to the measureable quantity - sales.
And again, this is not to say that a high quality product necessarily means high sales (especially when we talk about a "micro-scale" of quality). It also does not mean that low sales indicate low quality. It means exactly what it says.
I would posit, however, that "high rate of continued sale" (i.e., sales volume over time when removing the "spike" of sales in the first 30 days of release) is the
best indicator of quality - it indicates that a product stands the test of time and continues to appeal to gamers. That means it works on a macro-scale - lots of people are using it, and on a micro-scale - continued sales imply that many people find it quality and are finding it by word of mouth.
There's no "perfect" definition of quality, but again, I'm trying to reconcile two definitions taht may or may not work together.
--The Sigil