Proof (data access) of a confirmation pop-up online On Safari 11.0.3, I placed bids on an auction website a few days ago and after making each bid, I saw a confirmation pop-up that told me "Thank you for the offer". I thus have concluded the organization selling the items received my bids. 

However, I have learnt today that my bids were never received. Is there a way to prove that my bid was sent or that I indeed received a confirmation?

Is the information exchange (the form I filled online in a pop-up window and the confirmation I have received) stored somewhere in macOS?
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.