Well, you just learned that assumptions are unsafe. Even when buying new, you don't know something works until you yourself verify that it works. When buying used, this goes double - if not quadruple. Even though the merchant is honest and reputable, you cannot expect that every item for sale used has been thoroughly tested as it would be at the factory. Usually, it's been eyeballed for obvious flaws or damage and then put up for sale. My experience with the three lenses I mentioned was in dealing with reputable and honest sellers; one lens was returned as unacceptable, one was traded to a friend who didn't care about the unrepairable shutter (which is precisely how it was represented to me), and one was kept and is still used. Oddly enough, I bought this last one as a junk lens at the end of a camera show.
So the best practice is "trust but verify". And never ever use a piece of equipment, be it new or used, on a job until you yourself know it does what it's intended to do.
Lesson learned the hard way. I can't wait to get the camera and lens back from Eric.
I've used Pentaxes in the past. In my opinion the "star" Pentax lenses were those old Super Takumar lenses for the screw thread Spotmatics. I believe at some stage Pentax stopped making their own lenses and the production of their bayonet mount lenses was taken over by Tokina. At what stage this was I don't know. This is not to imply there was a drop in quality with the change, but those old screw thread Takumars had a certain quality that is not simply defined by lines per millimetre.