To prevent scamming a bit further I suggest that Gems can be bought and sold via the Trade Office.
This however creates one big problem. The free weekly gems and the levellingbonus gems. So, i suggest that BB changes the following things.

First we create a new type of gems. "Bound Gems". The BoundGems are linked to the account, you can use them, but not sell them. Yes, you can buy an goldparrot from them and sell those, but thats already possible. Normal Gems however can be bought and sold via the TradeOffice.

You get free gems when you buy them, also the gems created via the gempit are free gems.

An splitup between free gems, which you pay real life cash for and the boundgems which are present from BB will help both parties, first playes can use reallife cash to speed up their game without going to the loop of buying parrots or the items the player wants, secondly the number of support calls for BlueByte will stop as there is no need anymore to sell vouchers, which is clearly open for abuse.