I don't know if that could be a proof or not, but i also made some trades selling some digital goods at this forum tho. I don't know if it helps or not.
If there are posts for trades before the hack and you are still able to sign a message from it, it can be considered as proof of ownership.
Addresses should be bitcoin not Ethereum. Moderators and admins are keen on Bitcoin address and bitcoin signed message as proof. It is a bitcointalk forum, not Ethereumtalk forum.