Hi,
I’ve experienced this situation recently where a buyer placed the order without discussing with me and I had to cancel it because buyer provided very low quality files for me to work on which I couldn’t use to do what the buyer was looking for.
The cancellation affected my Completion Rate.
So I think there definitely should be a Button available for Sellers which allows them to Accept or Reject orders which are placed without discussion. This will protect Sellers from getting affected negatively for no reason.