Read
this thread about weight distribution hitches.
I'm surprised, anybody selling a trailer like that, would not tell the buyer that they needed a WD hitch.
Without a WD hitch, you're not only sagging in the rear and pointing your headlights in the trees, you are also taking a bunch of weight off your front axle. That will cause handling problems, which in turn can induce trailer swaying.
The primary function of a weight distributing hitch, is to restore some (in some cases all) of that weight to your front axle. While doing that, it also helps to level the combination truck and trailer.