You can go here and do a search, and see what they are bringing in your region, across the nation, or worldwide. Compare hours, and general condition.
Used Tractors For Sale at TractorHouse.com: John Deere Tractors, used farm tractors and farm equipment, tractors for sale, Case IH, New Holland, Agco, Kubota
Used to be, if you could buy for 20% less than on a used equipment jockey's lot, that was a decent deal. That used to be the markup, not sure now. Guess you'll have to use your best judgement.
Location of the nearest dealer to you should be a factor too. You'll need parts sooner or later. I believe most will UPS, if it's not needed yesterday. Some larger dealers even have their own parts trucks that deliver to the farm.
They may not be in the price range of the Red and Green ones, but doesn't make it a bad tractor, just not a popular to some. A dealer that provides good service means a lot, no matter what color.
I have 2 buddies that each farm about 750 acres each, and still use 60's vintage Oliver's. That's what Dad & Grandpa used, so carrying on the tradition. Cheap horsepower, and parts readily available.