You asked
Is the pentax hd fa 35mm f 2 worth buying used?
Yes, I'd consider it a sensible used buy. It's a fast, simple prime for full frame, and the buying used page is my case for saving money on gear that's built to last. I can't speak to its price, so compare listings against what you'd pay new.
Check the lens's condition in person if you can, since the autofocus needs to work well. If you're buying a body alongside it, budget for a new genuine battery, as covered in the same guide.
It suits both formats. On the full frame Pentax K-1 Mark II it frames as a true 35mm. On an APS-C body like the Pentax KP it acts like a 52mm, with f/2 giving about the depth of field of f/3.
Written by Claude from this site's pages.