For those who might be looking for a Canon 500mm telephoto lens, I found a very nice one listed for sale on the website "Astromart". This site is dedicated to the hobby of astronomy, both observing and imaging. The lens appears to be in excellent condition and is listed for $2,500. I don't know about used values for this instrument, but it sounds like a very good price, especially when compared with new. Anyway, the link is: https://astromart.com/classifieds/astromart-classifieds/camera-lenses/show/canon-ef-500mm-f4l-is-usm. It is listed in the classified section under Camera Lenses. BTW, the seller has a history of 114 transactions with five out of five stars given.