I don't know if they can be rolled back, but they can be disconnected and not running and then hooked back up before selling. I have a mower that the hour meter would not shut off and run the batt down. I did disconnect it, but not to be decieving.
I really do believe you can see a difference in the appearance on a machine that has 1000 or more hours on it then one with just a couple hundred. If it's been well kept a 2007 should look almost new. Value wise I'm not sure if you'll see a big difference in price with one having like 200 hours and one having 500 as both are low hours compared to the life of a diesel.
DsBota pretty much has nailed it right on the head and has a good point. For a tractor thats 4 years old I cant see many hours of use on it. Is the tractor for sale from a private owner or was it used in a business or on a farm?