Those who have never liked the fine distinctions of English spelling will rejoice to learn that the first recorded use of the word stationery, in Nathan Bailey's Universal Etymological English Dictionary of 1727, goes thus:
Stationary, Stationers Wares
The -ary -ery distinction was only introduced in the nineteenth century to annoy schoolchildren.
But what is it that makes stationery so stationary? Why so still?
Well, once upon a medieval time, most books were sold by itinerant tinkers and traders who would set up shop in a different place everyday, or often only on market day. An exception to this rule was the stationarius. This was an official position within a university. A stationarius was given a shop from which to sell books and parchment and the like. This meant that unlike most of his competitors he was a stationary booktrader and thus became known as a stationer. In return for the shop he was required to swear fealty and obedience to the institution.
The stationer only slowly became differentiated from the bookseller. It wasn't until 1656 that Blount wrote in his Glossographia
Stationer is often confounded with Book-seller, and sometimes with Book-binder; whereas they are three several Trades; the Stationer sells Paper and Paper-Books, Ink, Wax, etc. The Book-seller deals onely in printed Books, ready bound; and the Book-binder binds them, but sells not. Yet all three are of the Company of Stationers.
Which hardly clears things up. The distinction didn't really catch on until a hundred years later, and the -ery until a hundred years after that.