="Оля"Купи двое ножниц" sounds bad, but "купи одни ножницы" is very common, although itis not perfect from а grammatical point of view.
If neither variant is perfect, then, Оля, how would you say it?

To attempt an explanation of my corrections:

The inclusion of "it" in this instance occurs because in each clause you must have a subject. "It" is your subject in this case, as it refers back to something you mentioned earlier in the sentence.

Attempting an explanation of your mistake with the articles is slightly more difficult. "The" implies that you've mentioned "the grammatical point of view" before now. It seems to me that the definite article appears in English when you've mentioned the thing you're refering to before. Although there are probably other uses for it, too.

Джек