There are not really any web sites, publications, etc to determine tractor values like cars and trucks. There are some publications but not as close to real average prices like Kelly Bluebook, Edmunds and the NADA book for cars and trucks. Also the market for tractors is nowhere as big as the market for cars and trucks. Also there are not 30 to 100 used tractor lots in almost every community like for cars and trucks but new and used tractors are bought and sold daily, some where but no where near the volume of cars and trucks. Auctions are going on daily/weekly also for cars and trucks but maybe monthly some where on tractors. The I'll take and I'll give is the same process on tractors though. There are also dealers that don't want used tractors and there are dealers like Barlows that do want to buy used tractors and will pay a fair price whether straight out buying or taking in on trade.
I traded Barlows a used F2680e 60" deck last fall and he sold it. I bought a new F3080 and already owned another F3080. Now plan to sell (today) the first F3080 and maybe buying the F2680e with a near new 72" deck back since Barlows bought/traded it back from the man he sold it to in the spring or buying another new F3080. The price is good on the new F3080 but the price for the used under 200 hours F2680e 72" deck that I put most of the hours on is really drawing me that direction.
Bottom line, if your going to trade/sell to a dealer you have to find one that places value on used equipment and is secure enough in their own judgement to deal in used equipment. Some dealers are to insecure or to greedy to do other than new which they know the exact profit with low risk they are taking. Also most new Kubotas are not bought or owned by the dealer, they belong to Kubota and are placed on the Franchised dealers lot with dealer being told their actual cost the day they are sold so the dealer can decide how much they want to make before closing the deal. Different with used equipment. The dealer has to buy and pay for the used equipment out of their own pocket and hold it till it's sold so some dealers can't afford to buy the used equipment or if they do they won't pay much for it. Go to a buying dealer with money to deal with trading or selling used equipment.
Bottom line, there is a market for used tractors and implements but it has to be found and once found it is a moving (buying/selling) fairly active business but no where like the scale of cars and trucks but still there when you find it. My next purchase will be number 16 in 10 years so I'm not talking from theory or what someone told me or I read or heard.