I don't see anything that looks like a shear pin that is accessible on my Kuhn. At least nothing that one could get to without opening a gearbox, and nothing that looks like a baler setup.
Perhaps a key could be lost on a shaft, but that would indicate a lot of wear.
I've had Craigslist sellers underestimate the damage to their stuff, and the repairs necessary to fix them.
Have him tell you what turns and what doesn't turn, both turning the discs, and turning the PTO/pulleys. Does it catch anywhere when turning, or does everything turn smoothly? If one turns the disks 1/2 turn, then do the pulleys engage?
If there is a shear pin, he should be able to take a photo of the missing pin and prove that is it, or at least you should be able to find it on a parts diagram.
If this is being sold broken, then you need to price it as broken. Gears can be $200+ each, and you would probably need at least 2, if not several. Plus, of course, the labor to fix it.