I never said that they did, just that I believe that they were aware of them. Being under imperial employ from the start itself solves the issue of selling. Common knowledge of it is possible. Common knowledge of where to get it and how to get them is another matter entirely. Some strange, priceless creature might be in front of you and you might know what it is and what it does but that doesn't make the world and method of removal all of a sudden something that anyone would know.
Telling me that there are other methods is preaching to the choir. I suggest stuff because it seems fun, never because I think that there's only one way to do it.