require imediate payment on buy it now

I know you can still add that requirement if you already have an item listed.
Go to your active selling item list.
Go to "more actions" on the right.
Revise.
Scroll down to "Select how you'll be paid" (I know laughable isn't it?)
Click "add or remove options"
This will bring up a pop up page and you'll see where you can check a box to require immediate payment when using Buy it Now.

If you're beginning an item, it's still there under the PayPal section of Select how you'll be paid. You just have to check the box now.