* "Usted" is singular: It refers to "you" in the formal singular form.
* "Trabajan" is plural: It means "they work" or "you all work" (plural informal).
To correct the sentence, you need to make the verb agree with the subject:
* If you are talking to one person: "Usted trabaja en una papelería." (You work in a stationery store.)
* If you are talking to multiple people: "Ustedes trabajan en una papelería." (You all work in a stationery store.)
Important Note: "Papelería" is the standard spelling in Spanish.