Proof (data access) of a confirmation pop-up online
Safari on macOS has caching functionality that could potentially save copies of web sites on your hard drive - however, pages with dynamic content such as a order notifications, bidding confirmations, form content, etc. are generally not saved at all. I.e. the web sites forbids the browser from saving them to cache.
Therefore I doubt there's any chance to restore those pages for documentation. You would have needed to take screenshots, save PDFs or similar at the time the information was available to you.