I have been told the metal welds poorly.
And that it bends easily and stress fractures easily.
And that it has very little shearing or tensile strength.
It's basically soft and weak low quality metal.
Great for sign posts but not for much else.

The angle iron you refer to is most likely quality steel.
Unless you mean something like bed rails. They are soft too.
A trailer for use on the road needs good quality structural steel.

You don't want to have to explain a home made trailer to your
car insurance agent after and accident involving the trailer.
And you definitely don't want to have to explain where you
aquired the posts to the officer investigating the accident.

Pooh Bear

the sign posts were obtained legally their 1/4 inch thick and heavy as ****. not much weight difference than 1/4 angleiron. i can beat the angle iron and put a dent in it but the sign post takes a licking better than the angleiron

Could be a different kind of post than is used around here.
I remember posts from a long time ago that were tough and strong.
But there were so many problems with people stealing them
that they switched to the lighter more flimsy posts.
Hardly ever see any of the good posts anymore.

You could try some practice welding and see what happens.
Be well ventilated as the galvanizing will off gas bad stuff.

Call your insurance agent and ask him what he thinks
of a home made trailer that is not DOT certified.
Better to find out now than after the accident.

Pooh Bear