It's already been verified by Hilariousandco:
Op sent me the PM so I can confirm that's real at least.
Gleb verified himself after the PM in OP. Thus I'd say it's reasonable to assume they are still in control of their account.
I don't think he is. Whoever bought the account probably just asked him to send some proof to get out of the mess. Either way I don't really trust Gleb.
Bruno has contributed a fairly decent amount to the community over the years and based upon the fact he is (trying) to sell his accounts he likely is in need of money. So I would ask both hilarious and darkstar if they want to actively prevent Bruno from raising money he likely needs by selling something he duly owns?
Judging by the message, and the supplied information it has already been sold Otherwise, why would the PM be within the time frame of the password changes[1], and Gleb was likely willing to verify that he was still in control to protect the buyer.
Figure 1:
Quoting from seclog:
May 24, 2018, 08:19:02 PM - Gleb Gamow - password changed
May 24, 2018, 08:46:24 PM - Gleb Gamow - password changed
May 24, 2018, 10:40:27 PM - Gleb Gamow - password reset via email
May 26, 2018, 06:42:21 AM - Gleb Gamow - password reset via email
June 04, 2018, 01:44:22 AM - Gleb Gamow - password changed
And according to their trust page:
"This user's email address was changed recently."
Timestamp of the PM screenshot is right after the first password reset.