What you are actually buying
When you buy XAUUSD on a CFD account you are not buying gold. You are opening a contract with the broker that pays you the difference between the price when you open and the price when you close. That is the whole product, and it is why the position can be closed at any second the market is open, in any size, in either direction.
The practical consequence is that a sell is exactly as normal as a buy. There is nothing to borrow and nothing to deliver. If gold falls 10 dollars and you were short one ounce, ten dollars is credited to your account. If it rises, ten dollars leaves it.
The second consequence is that the broker is your counterparty for the price you see. Two brokers quoting the same instrument will not quote the identical number, and the difference between them is real money to you. That is why the conditions page matters more than the marketing page.
A CFD pays the difference between open and close. Selling is as ordinary as buying, and the price you get is your broker’s price, not the world’s.