That Ebay unit is an interesting multi use idea. Do search on boom poles and you will find lots of interesting ideas - including safety.
Speaking of safety, this design has a couple things that stick out to me. The lower part seems rugged enough - a box section with nice thick plate for the pins. I have a problem with the stress that the angle iron experiences when a boom is used. I don't like the idea of putting both bending and twisting forces and on that vertical section. What is also not present here that you see on all other booms is any bracing that triangulates the structure. I would not count on the top link to add any rigidity other than simple tension, like you would get having a chain or cable in place of the link.
Maybe the pro weldors here will comment that I am overly cautious. It's a clean design, and that top plate adds some twisting resistance, but I'd think about adding at least a couple more pieces of angle to triangulate the vertical structure, and even some pin-on braces that could run from welded-on tabs out on the boom to the plates where the 3-pt pins attach.
Having said that, I still think it's a neat idea as far as versatility goes. Better yet to make the whole thing out of square tubing, along the lines of what
Makers Mark did on his home made rake. Why not make this a 3-pt receiver hitch with the added provision for the boom - adding a lower reciever between the angles and on top of the square tubing to this would get you there, and those holes where the two lower balls mount could hold hooks, pins, or or clevis'.
