Here's an oldie but goodie: a Nikon TC-200 AI 2X Teleconverter still in the original box with the original instruction manual.
This device couples to any Nikon mount lens and doubles the focal length and the f-ratio. In other words, a 100mm f2.5 becomes a 200mm f5. It also works well for prime focus film astrophotography - attach the TC to the camera body and attach the T-ring to the TC and you've doubled the focal length of your telescope. I've used this on a C-5 with impressive results on the Moon.
$100 shipped in the USA. Insurance extra. Certified funds or Paypal plus 3%.
Thanks for looking and clear skies!