It's a really nice example. The Constantinopolis with two soldiers type was issued from Eastern mints, but not from Western mints...at least I haven't seen any that were not unofficial.
The seller you mentioned is well-known for his exuberant pricing, mostly on eBay. I have included a picture of his coin and the description of it. This coin is different from yours since the bust isn't wearing a helmet and doesn't have a reverse spear over the shoulder-- but certainly not worthy of the hyperbole and hefty price tag. The seller also doesn't seem to realize that the coin is not official.
"Unpublished, Unique (Early Prototype?). Size: 16mm, 2.03g. Professional Numismatic Notes: Around 330AD the mints of the Roman Empire began minting a series of coinage commemorating the founding of Constantinople, and also a series commemorating the existing capital of Rome. On these coins, typically you see a personification of Constantinople named Constantinopolis, and the reverse usually of a victory or soldiers and standards. This coin is a unique and heretofore unknown type in this series, featuring a Constantinian bust instead of the city deity. The legends are clearly that of a Constantinople commemorative. My hypothesis is that this coin may represent the earliest attempt at minting the commemorative series, but instead of utilizing a new obverse type they merely changed the legends of an existing type. This coin grades in good VF with a pleasing untouched and natural green and yellow patina. There is some patina flaws on the obverse, and a large surface scratch on the reverse. This may be the only chance you'll ever get to own this coin or a coin of this type in general!"
