Remember that a stationer sells stationery. Stationery is what you buy from Officeworks. Stationary is what the train is when it's waiting at the station. There's no such thing as a stationar.
Both words derive from the Latin stationem, which acquired the sense of a market stall. So you bought paper goods from a market that had stopped moving, at least for the time being.
And in case you don't believe me, here are some linky-dinks, with a little more back-story.