Had to go through similar recently.
I had a load of cards (18ish?) to sell, stuck them all for sale, but had to verify every single one, then got the email to verify, which took me back to the same page to tell me it was verified. Complete fucking waste of time.
After that, got told would be on hold for 2-3 weeks before going "for sale", by which point, they may be worth more but there`s fuck all I can do by then.
2-3 weeks later, a pop up on steam keeps appearing every half hour or so, before I finally stop clicking "x" & click "verify email". Low & behold, verify my email & ta-da, I get the pennies from the sold cards.
So, basically, they were holding that money until I verified my email, despite me having used that same email for years.
I have no intention of giving them my phone number, just so that it could get hacked/sold/posted publicly etc.
Anyhoe, fuck steam!!
P.S. Apparently you can use google authenticator instead of a mobile, for those that want to.