I can't.
There is no foolproof way to determine a multichar with 100% accuracy.
This *always* involves manual checks... (yes, *I* have to look into it)
So it is out of the question to allow trades. I can GUARANTEE you that >50% will be going to multichars to buff up.
As the gem transfer topic was open for discussion, a player did (so you might not say it is only *me* that thinks badly) a nice suggestion:
"Hey, why don't we block gem-withdrawals from multis?
I mean, you can simply ask a friend to withdraw them, he is no multi, put into HIS coffers, and you get them there? Why make it hard..."
So you will soon have a nice network that can circumvent any of this
And sorry, but I don't have sufficient staff that ONLY tracks abuses like this... which ARE hard to get.