Learn Italian: La bancarella è piena di roba vecchia e usata. - The stall is full of old, second-hand stuff.

In Italian, the adjective pieno (full) is followed by the preposition di to mean "full of". Also, notice that piena ends in -a here to agree with the feminine singular noun bancarella (stall).

Roba can mean a bunch of things, so why are the adjectives vecchia and usata singular?

Even though roba refers to a collection of items, it is grammatically a feminine singular noun (very similar to the word "stuff" in English). Because of this, any adjectives describing it must also be feminine and singular.

Is usata related to the verb usare (to use)?

Exactly! Usata is the past participle of the verb usare. In Italian, you can frequently use past participles as adjectives. In this context, it translates to "used" or "second-hand."

How would I say "old stuff to sell" using the grammar from this lesson?

If you wanted to add an action or purpose to the noun, you would use da plus the infinitive verb: roba vecchia da vendere (old stuff to sell). Our current sentence only uses regular adjectives to describe the stuff, which is why you don't see da here.