I spoke with the owner, Glenn, in Portland, and he is a very nice guy. He bought a bunch of stuff at the Wollard auction that he was selling.
Here are 2 pictures from Simplex manuals of trailers. One is just a frame that you build your own box for, the other is a factory built trailer to carry clothes.
In the 1948 manual, they also show a trailer with a prebuilt box, but I could not get a decent copy of that.
What I do notice about all the ones in Simplex literature is that they are round tube frame and built a lot like the 3 wheel trucks. The one on ebay is mostly flat plate, and the wheels mount very high, unlike the simplex ones.
If I had to guess, I would say built for something else and adapted to a simplex, but that is just pure speculation on my part.