Learn Portuguese (Portugal): Este agrafador é meu, mas os auscultadores são teus. - This stapler is mine, but the headphones are yours.

This ties directly into this lesson! When a possessive pronoun comes immediately after the verb ser (to be) to state who owns something (the predicative position), we generally drop the definite article. We say é meu to simply mean it is mine. If you added the article and said é o meu, it would change the nuance to mean it is the one that is mine, usually to distinguish it from a group of other staplers.

Are agrafador and auscultadores the standard words for stapler and headphones?

Yes, these are the standard everyday terms in European Portuguese. As a B2 learner, you might consume media from different regions, so it is good to know that these are distinctly European. In Brazil, you would hear grampeador and fone de ouvido instead.

Why use the demonstrative Este for the stapler, but just the definite article os for the headphones?

Using Este agrafador indicates the stapler is physically close to the speaker. Switching to just os auscultadores (the headphones) instead of esses auscultadores (those headphones) implies the headphones are the only ones relevant to the conversation right now, so they do not need to be pointed out with a demonstrative word.

Just to refresh, why is it teus if the word yours refers to a single person?