If you could lock and hide your own threads you would be able to unlock/unhide the threads that we moderators lock/hide.
You may not realize that we did it. You could think "I don't remember locking this, whatever." and unlock it.
Now we'd have to go back and lock your thread, notify you that we locked your thread and to not unlock it, and people would end up getting warning points over a silly mistake.
Sorry, moderators are here for a reason. If you want your thread locked, just click the report button. But in my opinion, locking is the final line of moderators saying "Your thread is done until further notice."
If any sponsor/donor could lock/unlock their threads, the meaning of it would change. We may lock someone's thread under suspicion of being hacked/attempting to scam (but not proven yet), and they could just unlock/unhide it, or people may think they themselves locked/hid it and message them anyway.
It just throws more confusion into the mix of whether or not a person can/should trade.