A lot of people want to buy/trade gems- most of the gems traded will have been bought by a player.
At this time you can only buy some gem items which can be gifted to you or traded directly though the new trade office but seeing the popularity of the gem vouchers it seems logical that this feature be implemented and should cost limited developer time to do it as well
To avoid abuse of this and/or account smurfing there should be some fee in it or penalty to it like you can buy a 100 gem voucher in the gemshop for 125 gems
Of course it would be better if you could buy gem vouchers (similar to sms vouchers) which can be redeemed in all countries (unlike the sms vouchers)
And best would be to have both options )