If it's clean that's actually below book by up to close to 10k depending on what it has/doesn't have. But that doesn't mean it's worth it. If it's from a dealer and has a warranty that helps, how all the fluids look helps too. Is front end tight or are joints ready to be replaced? Are they good units or were cheepos slapped on there? Anything with mileage like that, hopefully you'll be bringing to a mechanic to have it checked out thoroughly, REAL thoroughly.
I'm amazed what that mileage will "Book" at. KBB is stupid, but NADA still is up there. I checked it out and it does LOOK clean.
I have to say, that mileage is a put off, but for 4 years, that's for sure most all Highway which IS easier. These are meant to tow. If it was done without pushing it (Why you check it out) it won't be detrimental. I walked from super deals cause of a snowplow, but My truck had a 5th wheel and it was never a concern.
Does that guy deal high mileage trucks? He has a 320K 3500 too.
If it's what I was looking for, and I could afford a some wear items being replaced I would have it THOROUGHLY checked out, before just writing it off. But that's just me, I've had gassers I took well beyond 300K.