This dead horse clearly needs more flogging.
From my experience, there are about half a dozen "generations" of Model Ms: old, very heavy ones with wire stabilizers (IBM 1985-88), mid-era slightly modernized (IBM 1989-93) ones, mid-late 1990s Ms by Lexmark, and 2000s by Unicomp. I have owned every variation, and my current collection comprises about 2 each of "silver label" 1390131s, black label 1391401s, and blue label 1391401s.
Although I clearly like the old ones, most of mine have been substantially modded and would no longer be considered "typical" specimens of the genre. For one thing, I have bought a few recent boards for parts, such as a couple of barely-used M-122s, from which I harvested springs and other parts.
No one will dispute that build quality has slowly but steadily declined over the years, but there is great disagreement over how much and how significant this has been.
I have bought quite a number of IBMs on ebay, and at least 2/3 of them were good solid items. It is my habit to take them completely apart for cleaning and adjusting, right away, so ugliness and dirt do not particularly bother me. And I have spare keycaps, springs, and cables which are often damaged or missing.
I like the heaviest metal plates from the oldest boards, and I really like the square aluminum badges on the 1390131s, so that is what I rank highest. The 1391401 is surely the "classic" number and exists from near the beginning well into the early-mid 1990s Lexmark era. I would not want a 1390120 without the LEDs, but if you got one cheap, it could provide many good vintage parts for other projects.
Other, later part numbers are somewhat less pricey, but characteristics like lighter plates, attached cables, and single-piece keys do not really alter performance by all that much, even though you will hear it argued that they change the "feel" by a lot.
You hear the current Unicomps disparaged here, but they are perfectly good machines, and excellent value for the money. I would certainly recommend them to a newcomer.
The SSKs and "industrial" cases command very high premiums. The SSKs are different and many people consider a small footprint to be highly desirable, but the industrial gray color is simply a personal preference. The are very rare (probably well under 1% of the total Model M population) so they bring top dollar. And an industrial SSK (the Holy Grail) would surely fetch several hundreds at auction.
Back to your question: if you are asking whether the one that you see listed on ebay is "good" or "bad" look for some sort of statement about all keys tested and working. Or, what I do, bid on cheap ones and don't get bent out of shape if 10% turn out to be duds. Complete sets of keycaps sell for $30 from clickykeyboards or Unicomp, and most used Model Ms sell for between that price and twice that price. Plus a decent SDL cable will bring $10-12, so you will have some parts to keep or sell.
