Sheldon, good points on the market view, I wonder if used brokers like KEH will ever start to list that info? Also if the warranty period is still valid that adds quite a bit of value. I also had a new 70-200 but it failed after a couple of months of light use; had to be sent back twice to Canon service during the first year of use. Worked fine for 4 years after that - who knows.
Noah, tried to find any info on bad lens batches from Canon, could not find anything. And yes, never buy anything from a photographer on the seashore. I like salt on my food.
And Bradley, yes lol, number of shots for a lens is a very crude measure of a lens condition and wear, but we all have our guides. I still think condition is 95% of a lens value over any other measure.
Anyway, if you're buying local, take you matching camera body, test the lens, bring a flash light to look for mold or other possible interior aberrations, and like any used car, look for wear signs that don't match what your being told by the seller.
Regards,
Robert