Personally, I don't spend a whole lot of time worrying about whether my equipment will be legal in another state.
However, while there are Reciprocity Agreements in place between the states (most of them, at least), such agreements usually only apply to Drive Licenses, Insurance requirements, and registration. Equipment laws usually do not have reciprocity.
If it is an area of concern, one should NOT rely on what a salesperson has to say. Instead, check with the appropriate authorities in the state where you will be traveling and see what THEY say about the issue.
It IS, of course, highly unlikely that anybody will ever be stopped for a trailer brake check.
That 1900 pound trailer without brakes would be legal in Montana.