An office supply store sells stationery which means paper or writing materials.
When we say something is stationary, we usually mean that it doesn't move from one place to another. For example, a car can be stationary if it's stopped at a red light, but a person can also be stationary if they're sitting on the couch watching TV all day. It's like when you're standing in front of a tree and it looks like nothing is happening because neither you nor the tree are moving. In general, we use stationary to describe something that remains still, without going anywhere.
Not moving.
Example: The train remained stationary for a few moments, before lurching forward along the track.
