I've bought a couple of things from that same eBay
seller in Stepanakert. In fact, I have a cover very similar to yours: same stamps, but with a different section of the selvage inscription, postmarked 19 Nov 2018. Neither of the two envelopes that he sent me has any back stamp, though.
Here's one of the items that I bought from him:
Note the address: "Stepanakert, Artsakh, Armenia". But there's no other indication of routing through Armenia.