Could be a sting. Could be a misprint. Could be someone outside the US where the guns are legal. Could be a hoax. Could be a dealer sample G18. Pick one, or come up with your own.
As far as I know, there are no GLock 18s in the civilian registry, as this model was not made before the 1986 freeze. There are GLock 18s in the US, but their legal status is for sale only to Full Auto dealers, police and military.
This does not stop them from being stars on U-Tube videos, only from being owned by private collectors. Anyone can shoot one, with the owner's permission, you just can't own it in the US, unless you are in the right legal catagory.