Three stars were authorized for the Commanding General of the U.S. Army (though the central star usually should be larger than the others), both Halleck and McClellan serving on that post. And apparently some stretch the civil war until mid 1866 which would include Sherman as Lieutenant General as well.
Henry Halleck with three stars.
And initially shoulder boards were the standard for Confederate officers, just like the rank insignia, before the collar insignia became regulation, stemming from old army days and militia uniorms. With time the rank insignia changed (more or less twice but of course individual state forces at times had their own as well) and shoulder boards became less and less. Some even combined them, in some cases with multiple rank insignia systems as well.
An interesting case for that is Wade Hampton, combining the stree stars of a Confederate Colonel collar insignia with the Palmetto tree of a Colonel in the South Carolina Volunteer Forces.
Beside that the seller doesn´t say that it was actually worn, just that it was from the civil war. It could easily be a surplus made but never sold, or sold but just for teaching or display purposes (today many units and bases display all rank insignias, probably back then some units or institutions, more likely regulars, did so as well). I´m not saying that it is, I´m just saying that it could.
To put it in fewer words - the civil war can be pretty complicated. Still that price would be madness for an original, as would be putting in so few details ... however by now it has already quintupled and there is still a week to go, so we´ll see.