Learn Italian: Ho restituito l'elettrodomestico, perché a casa ne ho di robusti. - I returned the appliance because I have some sturdy ones at home.

This is a special rule for the partitive pronoun ne. When ne (meaning "some of them") is used with a descriptive adjective standing alone (like "sturdy ones"), you must put di before the adjective. So, "I have some sturdy ones" translates to ne ho di robusti.

What exactly is ne replacing in this sentence?

It replaces the plural noun elettrodomestici (appliances). Even though the first half of the sentence uses the singular l'elettrodomestico, the context of having "some" at home logically shifts the reference to the plural form.

Could I just leave out ne and di and say ho robusti?

No, in Italian you can't use an adjective completely on its own to mean "sturdy ones" as the direct object here. You have to specify what you have. Since you aren't repeating the noun, you must use ne to represent "of them," which then requires you to use di before the adjective: ne ho di robusti.

Why use restituito instead of ritornato for "returned"?