Learn Icelandic: Ég sé engan verðmiða, getum við prúttað?

It is in the accusative case because it is the direct object of the verb sjá (to see). Since the noun it modifies, verðmiði (price tag), is masculine, the pronoun enginn must take the masculine accusative form engan.

Why does verðmiða end in an -a?

The dictionary form is verðmiði, which is a weak masculine noun. Weak masculine nouns ending in -i change their ending to -a in the accusative, dative, and genitive cases. Since it is the object being seen, it takes the accusative case here.

If the shopkeeper already knows I am looking for a price tag, could I drop the noun and just say Ég sé engan?

Yes! Similar to how you can use adjectives independently by dropping the noun (like saying þessi stóri for 'this big one'), you can do the same with enginn. Ég sé engan translates to 'I see none' or 'I do not see one'. The word engan stays in the masculine accusative form to refer back to the dropped masculine noun verðmiði.

Why do we use the form prúttað instead of the infinitive prútta after getum?