There's no one answer that would work for all people. A used trailer can be a bargain based on price, but the purchaser has to have enough knowledge to really check out the unit for any problems, especially dreaded water leaks and resulting rot, delamination, etc. If you know a RV service tech or facility that you can trust, you can arrange to have a unit inspected, just as you would with a used vehicle. And, if all goes well, you can save $$ by buying used, possibly buying a better trailer than you could afford if you were buyinng new. But, once you buy it, any problems that arise are your headache. How big of a headache will depend on whether you can do a lot of fixing or repairing. If you have to have someone else work on it...you know that with today's shop rates, it's going to cost.
Buying new gives you the advantage of a warranty. How much of an advantage depends on the warranty itself and the support provided by the dealer where it was purchased, or the dealership that will provide warranty service for that specific brand of trailer.
Our first Starcraft pop-up was purchased used and it was fantastic. But we only used it for a year, and decided that it was too small. Second popup was purchased new. It had two major problems...axle and roof failure. Starcraft and local dealer were fantastic at getting it fixed. Third popup was new, and I don't think we ever had any warranty issues, and served us well for 10 years.
Our Sunline TR280 was purchased new, and only had a couple of warranty issues. Again, the dealership was A-1 in taking care of the problems...biggest one was a leaky window.
Sooo....bottom line is that their are pros and cons to buying used or new. What's best for a new buyer really can't be determined by anyone except the buyer. If you lack knowledge and hate any risk, purchasing new may be the best route. If you know what to look our for, and can stand a bit of a risk, a used trailer can provide a lot of $$ savings, and the small warranty issues may have already been taken care of. So...how do any of us advise someone else aside from telling them to do as much research as possible and learn what to check.