I think there should be a distinction made between hobbyists that set up shop and sell fish from their home vs. storefront and/or warehouse operations. The later would tend to participate on this for the sole purpose of generating SPAM. Whereas the hobbyist is here to learn and contribute, and occasionally mention their wares in response to specific request.
I think this is where the distinction should be made. If a list member asks if anyone has a specific fish, book, etc., and "commercial" list members respond along with the casual hobbyist, I think no one would object. The thread on TDS testers is a good example. No one SPAMed the list with an offer to sell the product, rather a list member was inquiring where to find them.
I personally would also welcome periodic announcements from "commercial" list members when ever they have the good fortune of receiving a shipment of Apistos or a recent spawn grown out to shipping size available for sale. But that's just my personal opinion. Others may wish to restrict content more rigorously.