"They buy the other suits."

[Brian625246]

Why does cumpara costumele celalte work? How would i know it's they? I could buy the other suits.

Eu cumpara costumele celalte. Ei cumpara costumele celalte. Assuming those two sentences are correct grammatically, how would i know in the original sentence that this is not he or she buying the other suits

[potestasity]

I'm not sure I understand your question. Verb conjugation makes it clear that you can't use "eu"; that would be: "eu cumpăr costumele celălalte".