I believe BB do not want players selling vouchers for in-game resource, because of the very fact that players then do not have to buy vouchers themselves. It DOES reduce sales of vouchers/gems, as has been discovered in the past, and in other games.
The trade shop has some items that can be gifted, and some that cannot, to force players to spend money to get the gems. Not to spend resources to get gems.
I believe the main reason BB wish to give bans for selling vouchers on main chat is simply to keep it OFF public sales. They do not support it, and would prefer it didn't happen at all. but to issue a complete ban on it would mean they must police it, which is a HUGE undertaking. They have no intention of trying to police it.
But they will discourage it by any simple means, such as banning from chat.
It's a compromise, until they introduce a way to eliminate it from happening.
So, if you want to sell vouchers, then you do so at our own risk... If you do it obviously and in a way that is easily provable, BB will not hesitate to punish you. And while BB won't go out of their way to investigate/prosecute. They also won't give a toss if you get scammed, in fact they'll secretly smile and hope it deters you doing it again.
And that goes for anything that BB have not designed an in-game way to trade. They do not wish to have to give you support for being scammed on anything. so they say they don't support it, and it's at your own risk. Dont do it until BB have made an in-game way to do it safely.