I'll think about it and come back tomorrow — will you hold it?
ri-SER-va — three syllables; stress on the second. 'Riserva' as a verb is third-person singular (you hold/reserve).
Use this when you are interested but not ready to buy immediately. Sellers may or may not hold items — some will, others will say the market is first-come-first-served.
'Ci penso' (I think about it) is idiomatic — literally 'I think about it there'. 'Torno' (I'll return) is present used as future. 'Lo riserva?' (will you hold it?) — 'riserva' is present used as polite request. The answer depends on the seller's policy and how much they believe you will return.
Se lasci un acconto, lo tengo da parte.
If you leave a down payment, I'll set it aside.
The seller offering a condition for holding — common response
Posso avere il suo numero di telefono per avvertirla se si vende?
Can I have your phone number so you can let me know if it sells?
Asking for contact so you can be notified — shows genuine interest
Quanto tempo me lo riserva?
How long will you hold it for me?
Specifying the hold duration — sellers usually set a limit
Italian antique market sellers are typically at the market only on specific days — many Porta Portese sellers come to Rome's famous Sunday market from other regions. Missing the seller means missing the piece. Leaving a caparra is the most reliable way to secure an item across days or weeks.