I guess there were no easy way to prove the ownership, even if Google+ come and answer here, it'll be a classic he-said-she-said.
I contacted Google+ via PM pointing to this discussion and since he replied, I assume he knows about this thread.
He always can sign messages on some of his previously used BTC addresses to prove ownership of his account.
Buying account and selling of account is against the forum rules
Where did you see that rule?
18. Having multiple accounts and account sales are allowed, but account sales are discouraged.