1. There can be scams. Deals too good to be true aren't true. Usually ask them for photos of something specific and obscure that you happen to have interest in - if they can't produce the photo they don't have the tractor
2. There's lots of stories about how few hours are just as bad as lots of hours. I don't think they are, particularly at 3-5 years. If it hasn't been sitting outside and hasn't been abused it'll probably be good buying.
3. It's not that easy to manipulate hour meters, and even if they have, the rest of the tractor either looks almost new or doesn't. Condition matters a lot irrespective of hours.
Some will be scams, but some will be real. People buy tractors then circumstances change, or they just have a lot of money and happily bought a machine they only use a little bit. If the machine is real, and the hours are real, and it's in good condition, then it's probably great buying.