With all of us hobbyists constantly tweaking and sometimes upgrading our systems, I would expect there would be some very good used equipment out there, but I don't know where to find it reliably. For example, I am wanting to upgrade my Thiel 1.6 to the Thiel 2.4. I will want to sell my old ones. I would also like to find used 2.4's. In neither case do I have any idea where to turn for the best outcome.
So I am looking for recommendations for trustworthy sources for used stuff. Ideas?