This sentence is unclear. In Polish pronounciation, the Ę at the end of "Kupuję" is not voiced nasally. Therefore, it is not possible to ascertain whether it is first person "I" or third person "he/she". In this case, a pronoun such as "JA kupuję" or "On/ona kupuje" should be added to the sound file in order to be clear.
The assumption that ogonek (that hook sign) is marking the nasals is a simplification. The exact pronunciation "rules" are pretty complex and it may differ noticeably from region to region. Sometimes there is no actual nasal vowel there, and that doesn't only happen at the word end.
The simplified pronunciation rules would say that you should attempt to make that nasal sound and if you won't get exact, it's still okay.