I got this from Ebay, near mint, for £30 including postage. No lens cap or case, but at that price who cares
It is a bit plasticky, as the reviews here say, but the optics seem very good. The only downsides are the slow aperture, a bit of focus hunting, and that it dates from from Pentax's "Silver Period" of styling so it looks a bit odd on my K-1. Sorry about the lousy quality picture of it on the K-1 - I had to use an old P&S but you can see the point about styling.
Even the mount is plastic, and I am wondering if I could replace it with a metal one canabalised from a lens sold on Ebay for spares and repairs.
I had not realised that lenses from the end of the film era, with KAF mounts, work fully on a modern DSLR. It is just a matter of putting the aperture ring on "A". I think I might look out for some more. I did a shoot with an old test chart and at 180mm focal length and full aperture it resolved the finest test lines in the corners of the chart. It's a wolf in sheep's clothing! I have done a lens review here
SMC Pentax-FA 100-300mm F4.7-5.8 Reviews - FA Zoom Lenses - Pentax Lens Reviews & Lens Database with one of the test pictures.