I once got a nice looking 180-600 mm f/8 ED Nikkor as "junk" because it could not focus at all. The zoom ring was stuck and so was the focusing collar. (this is a slide zoom design). With the lens mounted to the camera, no image sharpness at all was found at any distance. Obviously the lens had been tampered with as witnessed by the damaged screw hole in the tripod foot and tell-tale marks from screwdrivers here and there However, at $100 incl. shipping for this elusive and huge lens, who should complain. It could be stood on a shelf as a trophy if nothing else.
Fixing the tripod mount was easy by drilling and tapping a hole for 3/8" instead of the tiny 1/4" stock screw mount. Had to go to this size because the damage was comprehensive, but the foot for once is a massive slab of metal so plenty estate to work with.
When the lens was opened, the reason for it being jammed was obvious: the 'repair tech' had matched the helicoid in error and it jammed against the end stop. Also an easy fix. The guides for the zoom control were tightened too much.
However, although these obstacles were resolved, the lens still was unable to provide a focused image. Patiently working by flipping element after element, the culprit was found to be a wrongly orientated element #8 (or was it #9) deep inside the lens assembly. Each 'flip-over' necessitated a lens tear-down then re-assembly, so getting there took 'some' time.
Then, all that remained was adding a CPU and I got a super zoom for next to nothing. Although the 180-600 is big and heavy, it is dwarfed by its bigger brother, the mighty 360-1200 Nikkor. Both share the same high degree of colour correction so they lack the IR focusing mark.