It seems like there predominately are two kinds of used 660's that I see. Either ones that are in near new condition, or those that are completely beat to crap. The beat to crap ones are usually from loggers, and most of these are project saws. Or maybe they sat in the back of an Arborist's truck for years, used for the occasional large tree, with less internal wear but cosmetically thrashed.
The other kind come from non-pro users; guys who bought the biggest and best saw they could, and either never used them or hardly ever. These are often babied, and put on the shelf next to the rarely used professional tool set. I picked up a used 066R with a 32" bar with two new chains, and some gear last year, that the guy cut down 3 trees (according to him), then drained it and put it on the shelf for 10 years. It looked 99.5% new, as in no paint wear at all, even on the bottom. I could have eaten off of the clutch, it was that clean. I gave him $700. A new 660R with a 32" bar is $1000 plus tax here, so best case you are close to $1100. For me at the time, I needed another big saw, and wanted to save the $400 (
plus I always thought the old 066's ran better than the newer 660's :msp_thumbup.So, I would agree that if it is in truly like new condition, $750 or so is a fair deal. More than that and I'd walk. Maybe the guy with the saw has some goodies to throw in to sweeten the deal.
It's important to remember that most folks here can get saws for under list, so we're likely to pay less for a used saw. My guess is that if you wanted to pony up for a new one, some site member could point you to a dealer in your area who could cut you a deal.
Good Luck!