How to create a sending address with Bitcoin Core? I downloaded Bitcoin Core and I'm just having a look around and I can't for the life of me figure out how to create a sending address.

I'm using a site to buy stuff and to register I need to "Setup your payout addresses" which is "the address where centralised escrow is released to the seller, and where buyer refunds are sent".

So, what do I do?
You are not looking for a sending address but for a receiving address! 

Coins always come from the address that you received them to, so one doesn't need special "sending addresses". Your purpose here is to enable the escrow to send you a refund in case something goes wrong with the purchase.

To find out your address, switch to the "Receive" tab:



The "Receive" tab will show you a list of all your addresses. The list will be empty at first, so you have to click "Request payment", which will open a new window showing your address:


You'll want to copy the alphanumeric string after "Address:" in the last line. To show the address again after closing the window, you can double-click the line in the list, or select it and click the "Show" button in the bottom left corner.