How do i show proof of ownership??

[ExPi]

So, I was going to sell some of my CS: GO loot boxes because I wanted to buy some games. But email verification is a pain in my ass, so I was setting my steam guard on my phone. But I don't have my old phone, nor do I have written proof of payment because it was years and thru a third party. I still have my old email and I know my account ID and Password is there any way of proving it?

[ReBoot]

https://www.bing.com/search?q=steam+account+proof+of+ownership
First search result

[Jerry]

When setting up your phone, you received a revocation code. Use it.