I'm torn on this one-I have a trailer that I bought sight unseen used-9 years old-and it's a solid rock-and I believe it was used a ton! I've bought new and used-both have there advantages.
Now ideally you'll find something that's older and was stored in a climate controlled barn and hardly used at all-but the seller of that unit will not be the rushed liquidator that my trailer came from! The major components are what you should be most worried about and some A/Cs last 5 years and some 20-so it can be a toss up-I've become more concerned with fit and finish-frame rust-any chance the tires might be brand new-things like that. No warrantee on old-used either, but after the first year many things are not covered anyhow. One more thing-there is a gigantic difference in build quality-even amongst some of the higher end builders-find a good unit and it will last a very long time!
I think that you just need to shop hard-super criticize the units you like-and BUY!
OH-HEY-WELCOME TO THE FORUM!