Learn Italian: Che cosa comprate oggi in negozio? - What are you all buying today in the shop?

Che cosa literally means 'what thing' and is the traditional, fully correct way to ask 'what?'. In everyday spoken Italian, people often shorten it to just cosa or che, but all three are perfectly fine and mean exactly the same thing in this sentence.

How do we know it means 'you all' are buying, since there is no word for 'you'?

The verb comprate comes from comprare (to buy). Its -ate ending specifically matches voi (the plural 'you'). Because the ending makes the subject completely obvious, Italian simply drops the word voi.

The translation says 'are you all buying', so why isn't there a word for 'are' before the verb?

Italian doesn't use a separate 'to be' verb for actions happening right now the way English does. The single present tense verb comprate covers both 'you all buy' and 'you all are buying'.

Why is it in negozio instead of nel negozio (in the shop)?

For certain common everyday locations like shops, pharmacies, or the bank, Italian often uses the preposition in without a definite article ('the') when talking about being there generally. You can just say in negozio rather than combining in and il.