I'd offer the seller $1750.00 because if you're going to sell the lens in a year, you'd probably ask $1950 given the condition. Forget about Canon repairing anything because Canon does not repair the 500 f/4.5 anymore. Maybe local repair shops in your area may have parts like the DOF window. Also, the 500 f/4.5 was focus by wire. If you decide to buy the lens, make certain that the focus motor is working and doesn't sound funny if it does fire up. Finding the focus motor would be near impossible and the 500 f/4.5 can't be focused manually if the focus by wire motor if damaged.
Whenevr I buy anything used, I always ask myself how difficult will it be to sell in a year or two. The cosmetic condition of the used item does play a part in determining saleability.
Good luck, I wouldn't buy this lens.