With trailers look at the fit and finish. Welds, wiring runs, lights, paint job (look under the trailer), ect. That will tell you a lot.
Lets face it, there are only so many ways to build a trailer and most are built 95% the same. The only real differences are in the way the extras are done like the way ramps store, where the jack is place (set back is the best so you can lower a tailgate on the truck or open the rear hatch of a SUV), ect.
Its the little things that cost money. Much like buying a house or a truck. The little things make it or brake it like the options of crown molding, marble counter tops, hard wood floors, or in the case of a truck the big engine, auto tranny, 4x4, GPS, ect.
Chris