Do unlocked iphones need a sim card?

I'm planning on buying an iphone 4s and using it as an ipod but the person selling say "no sim card". Will it still work?

It should still work as a music player, yes.

Buy an iPod touch to be an iPod touch, not an iPhone.

It would need a SIM card, at least for initial activation because at the end of the day it is a phone & people aren't (or shouldn't) be selling iPhones with SIM cards anyway since the SIM contains their number for their carrier.

To make phone calls, send text messages or access none Wi-Fi data you need a SIM card.