Learn Italian: Ne abbiamo comprati di scontati. - We bought some discounted ones.

This is a special rule for the pronoun ne. When you use ne ("some of them") and want to describe the specific ones you are referring to with an adjective, you must place di right before that adjective. It translates to the English "...ones", as in "discounted ones".

Why does the verb end in -i (comprati) instead of the usual -o (comprato)?

In the passato prossimo (a compound past tense), when ne is used to represent a part of a quantity ("some"), the past participle must agree in gender and number with the noun that ne replaces. The ending -i on both comprati and scontati tells us the items they bought are masculine and plural (for example, libri or vestiti).

Could I say this without ne, like Abbiamo comprato di scontati?

No, that wouldn't make sense in Italian. The di + adjective structure relies entirely on ne being in the sentence. If you wanted to drop ne, you would have to use a regular noun and a partitive article (like dei), for example: Abbiamo comprato dei vestiti scontati (We bought some discounted clothes).

What if the things we bought were feminine, like shirts (le camicie)?