I'll come back tomorrow — can you set it aside for me?
TOR-no do-MA-ni lo TYE-ne da PAR-te
Asking the vendor to reserve a product for collection the next day.
This is a phrase used by regular market customers who have built a relationship with their vendor. The practice of tenere da parte (setting aside) is common for seasonal items in limited supply, such as truffles, porcini, or artisan cheese.
Me lo metta da parte, torno nel pomeriggio.
Put it aside for me, I'll be back this afternoon.
Returning the same day
Le telefono domani mattina.
I'll phone you tomorrow morning.
Confirming by phone — requires having the vendor's number
Quanti ne ha? Li prendo tutti.
How many do you have? I'll take them all.
Alternatively, buying the entire stock on the spot
The personal relationship between Italian market vendor and regular customer (cliente di fiducia) is one of the pillars of Italian food culture. Vendors remember preferences, save the best items, and often give small gifts (un cadeau) with a large purchase.