“Stationary” means “not moving” “staying in the same place.” Hundreds of years ago, there were no shops like today. Merchants traveled around and sold things in one place for a short time. When permanent shops started in England in the 1300s, they were called “stationers” to show that they were always in the same location.
Over time, the meaning of “stationer” narrowed to mean only a shop that sold books and writing materials. The things a stationer sold were called “stationery”. We still use this word to refer to materials used for writing, especially for writing and sending letters through the mail.