I'm still new to the site but I'll throw my 2 cents in. I've rebuilt and ran several 064's and 066's and like them both. If it's completely rebuilt and shiny I would say that $750 would be reasonable, but not for a used one. To me 600 is also a little high for a used one but if he fixed it up and did a good job I would be comfortable with the price. Don't get me wrong the 064's and 066's are great saws and I would use either before picking up a new ms660 or ms661 but like another member already mentioned the 064's are getting a little old. However, they aren't that old where they can't be reliable work saws, both of the duty stations on my district have 064's for large hazard tree felling and they run great and are early 90's saws. If that saw hasn't been rebuilt then I would be very wary of the bearings and seals starting to going bad, worn piston rings(low compression) and a gunked up carburetor. Probably a good bet that the clutch springs are pretty worn too. I would also be wary of how many aftermarket parts are on it, some aren't bad but there are a lot of junk Chinese parts being sold on eBay nowadays. Immediately from the picture you can see an aftermarket clutch cover so I would check with the guy on stuff like that. Personally for a used good running 064 I would be more comfortable in the 400-500 dollar range.