This is mostly speculative, but seems logical to me:
1. It would be have more of an indirect impact, but I avoid shipping liquids as much as possible. Water is heavy and takes up space, which means more energy and more vehicles needed to move the items. Think about how much space and how many containers you would need to ship 150-200 gallons of saltwater compared to buying a box of your preferred mix and using the water you have at home. You would practically need a separate delivery vehicle for your one order. The same can apply to supplements like All For Reef, two part, etc. That's a lot of wasted fuel, plastic, and possibly an entire vehicle that could be making multiple deliveries instead of just one.
2. The same goes for livestock, which could have more of a direct impact. That's a lot of water being moved around. It is also more stressful on the fish/coral/invert, which could lead to unnecessary loss (subsequently having to repeat the process over again, consuming even more resources). There is also a lot of plastic waste that could end up back in the ocean, everything is bagged up 2-4 times in what is usually single use plastics. I try to reuse specimen containers or other jars when buying, selling, or trading-in. I also add to any local sale ads that if they bring their own containers then I will throw in an extra freebie.
3. GOOD HUSBANDRY! Make sure your system is healthy enough to support the animal you are buying. Stop buying livestock if you have no clue what it is, and therefore don't know its care requirements. Buy animals that are proven to thrive in captivity, are suitable for your tank and equipment, and compatible with the other inhabitants. Needing to constantly replace wild collected animals that you killed because you didn't provide the proper environment is going to have a direct and unnecessary impact. Even if it is a fraction of a percentage of the animals in the reef, that is still another animal that shouldn't have been taken from the reef.
I don't think any of that would really require much effort on our part. We should be practicing good husbandry anyway and most of my suggestions would end up being more economical for the hobbyist whether it is from cheaper dry goods or less money spent replacing livestock.
I also think more pressure should be put on the livestock supply chain. There are videos of businesses receiving shipments and saying that they expect XX% loss on every order. That can add up if there is an importer, possibly an additional wholesaler, and the retailer all anticipating a loss. If each step of the process is experiencing 10-25% loss from shipping that is 30-50% more livestock that needs to be collected.
That was way more than I expected to say on the topic.