Ok, mystery solved, I think...
I did their 2FA for my phone number by requesting a code and entering it after they texted it to me. It appears I am now able to respond to messages.
If that was the whole issue, then no problem but their communication was odd in that their message requested "a verified phone number and payment method" yet said nothing about 2FA, which was in a different message that explained the purpose was related to "buyers and sellers". Had I realized all they were asking for in order for me to message was a 2FA on my phone number, then they should have simply asked for that and I would have proceeded and never would have posted here.