'You name it', is usually when you're telling the person what you want or need.
For Example if you (Lee) said to me: 'Jennifer I need a favor.'
I would reply: 'You name it!'
This reply means that whatever you are going to ask me to do, I will do it; just tell me more about what you need me to do.
Another Example: If I were looking to purchase a car but only had a budget of $3000. Then I would walk up to the car salesman and say:
'I'm looking to buy a car but I have a small budget.'
The salesman would reply, "Okay, you name it."
The saleman's reply is like saying Okay, tell me more about what you want.