Do i need a nano sim for the boost mobile iphone 5s?

I'm buying a iphone 5s for boost off ebay i was wondering do i need to buy the sim card to? Even tho boost mobile is a cdma network do i need to purchase the nano sim card to activate the phone?

Yes it needs a nano SIM card.
And if you have an card which you want to transfer it on iPhone than approach the nearest cell phone retailer and ask them to cut the sim card.
They will do it.
So you will no be having to purchase a new sim

No need to get a SIM as there's none. Just get the phone activated on your number.