All the concerns brought up about the age of the trailer are universal - and would apply should you be looking at the same unit as used.
In my mind, there is still value in it being "new". Sure, inspect the roof, tires, and everything else (like you should before any purchase). Also verify the warranty.
If all checks out, I would figure out what similar used trailers are really selling for (around here, book value isn't accurate to sales prices), and offer 20% more. Go up some from there.
That dealer is in a tough spot, kind of surprised they haven't wholesaled the unit yet.
