How are big sites able to vouche their customers 24/7?

When I trade big gold sellers they usually let their workers vouch on their own account.

 

A few methods the bigger sites or company /workers have.

For instance Bogla has workers who have Bogla accounts, same as Divica, therefore people acknowledge that they are apart of the company and have individual reputation. However a livechat (I believe) work as a pm.

Someone such as Won_Gold would acknowledged who works for them and give express notification in their gold selling etc shop/thread of who their worker is specifically and that they have right to confirm pm/vouches etc on their behalf.

These methods work for bigger sites as they have reputation, and in case worker scams they would be liable.

If your customer specifically want your pm as confirmation, but your willing a worker to give that pm on your behalf, then either way your accountable.