At the very least youd need a signed message from the sending address I would think.
Why?
Every adress is unique for each user.
We are commonly asked how we know that you sent a payment. The address above was newly generated just for you, so when any BTC is sent to it, we will know to credit it to your account. It is all automatic.
It seems
highly unlikely that someone else send bitcoin to (
it) -> referring to his adress specified for his Copper Membership? I'm pretty sure theymos could check whether or not that adress is connected to his account.
The only way (As far as i know) that that would be possible if is someone else also had access to his account.
I sent it from a trading floor
Which means that he most likely doesn't have access to the keys belonging to the adress.