I thought a very few were imported into the US to try to get LE/Military sales. I suppose a Class 3 Dealer might have gotten access to one or two. This is all speculation on my part though. $900 seems cheap to me too.
^^This^^
It is a post dealer sample made after may of 1986. That's about the going rate for a Glock 18. You must have a vaild SOT (class 3 as he calls it) and a Demo Letter from a CLEO to transfer it to you.