After purchasing our 5er in August we had a bit of a situation. Not too sure if I should "panic" now or not

I was thinking about this and thought maybe I should run this past my friends here before the towing season begins
We are brand new to 5ers. We stopped at an abandoned gas station to try "jacking" the trailer to see if it would hit the cab of PU. Looked like it might so we set our Curtis hitch to "slide" but the trailer would not lock up. The trailer kept coming along. The truck was brand new with a built in brake controller. That was fine.
When we got home had to chock the trailer in order to dislodge. Son in law came over later, set the brakes on the trailer. We didn't hook up again but just tested by spinning the tire and things seem to be okay.
Now my fear...how could the previous owner not know this would happen. The brakes not set ?? They said they hauled it very little.

Apparently they bought it new.. They had a long box PU so no need to use slider but shouldn't the brakes have been set better ?? Wouldn't he notice this ??
The sellers were semi retired, own their own business and seemed quite honest. Had a beautiful home and just purchased a lovely motor home thus selling their 5er. I didn't get any bad "vibes" from them but ..........
This situation has been nagging at me all winter. Just sitting in the back of my head coming to mind on and off
Any idea as to how this happened. Think it could be prelude to other problems ??
Thanking you in advance for thoughts !!
Jo