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This (
Tokina at x Tokina SD 150 500mm F 5 6 SD MF Pentax K Mount Lens | eBay ) might be a good deal. The ebay listing is a bit scanty on detailed condition info, although there is some info. [The generic eBay canned description says "for Nikon", but it's obviously a Ka-mount lens.]
I noticed that the drop-in filter holder seems to be "dropped in" backwards, so I would suggest asking the seller if there is a clear filter in its tray, and if it indeed is in backwards (although the filter tray may be "centered" in such a way that it doesn't matter - dunno - don't remember). It wouldn't hurt to get some more specific descriptive info (on glass, aperture blades, etc.) in any event.
I used to have one of these many moons ago (back in Ye Olde Film Days of Yore), and, while it is very well built, I found that it (my copy, anyway) was a bit soft at longer FL's. (My own review is at
Tokina AT-X 150-500MM f/5.6 Lens Reviews - Tokina Lenses - Pentax Lens Review Database along with one other, more positive review.)
I do think that $280 is a good price ~IF~ the lens is complete and ~IF~ it is indeed in great shape.