My 2cents...
I bought new, and probably will never do so again. The warranty headaches are a PITA, should anything need repair (and almost a guarantee there will be needed repairs).
That said, not sure I would buy used from a dealer, as they probably have no clue about the history of the trailer, and for the sake of a sale, might make shoddy repairs just to hide damage, or don't disclose problems, etc. Not saying don't do, just not sure it would be for me.
Think I would buy from a private seller. One that doesn't seem to have anything to hide and doesn't mind you poking around (their body language/words may speak volumes if they are hiding something) such as removing panels and things to see beyond.
I routinely remove panels and things to look inside for water damage or other problems on my own trailer, so as to head off a big problem down the road. Actually discovered a leak behind some false cabinet panels and I was able to fix it before any serious damage occurred. Had a p trap come loose and fall off the drain on an old trailer I had; flooded things badly and resulted in a soft floor pretty quickly. (only reason I knew I had a leak was the water pouring out the bottom of the trailer-not sure how many showers I took before I discovered it)
Part of the reason why I now poke around routinely looking for things like that.
Any way you go it's a gamble. But lots of good advice here from others so hopefully you'll have all the bases covered.