I know this as I know that no vendors have been prosecuted for selling aya ingredients. It all has to do with how police works; basically they do not enquire something unless a complain is made and in the case of aya one will not really hear any complain. With complain I mean some anonymous tip or neighbours getting suspicious over crazy sounds in the middle of the night from the next flat, too much traffic in the house, whatever.

The point is that they can press charges to anyone preparing aya if they wish; it is simply because it involves a class A substance. But the fact that this substance is very low in their list plus there no complains from other people as well, police and prosecutors do not really bother too much.