Most fish in the wild are endangered but redtail sharks are not endangered. They are regularly bred in fish farms around the world and have been for years.
Re: collectors. They want different and unusual so they can show it off to their friends and so they have the only one in existence (not quite the only one, but that's the gist of it).
We used to have people coming in looking for rare (usually aggressive) species of fish, most of which we didn't carry. My best friend kept bugging me to get in a group of piranha for him. He wanted to let them go in the local river
He never got them.
As a collector myself, I can say that you are always after that one thing you don't have. Part of the fun of collecting is looking for that one item. It might take you years to get but you keep looking until you get it. Then you move onto the next item and so on. I collect Hippeastrum plants and had a huge collection. I lost them in 2016 but have started to rebuild the collection and every year when bulbs become available I go hunting for ones I don't have.
Even though I am a collector, I also like to share what I have. I breed the plants and when I get seeds or spare bulbs/ plants, I give them to other people that are interested in them. I used to do this with an English Oak tree we had in the backyard. Each year I gathered the acorns and planted them in pots containing soil. Every spring they would pop up and I would give oak trees to people that wanted them. When we moved out of that place, I never managed to get a cutting of the tree for myself. A few years later I went back looking to get cuttings but the new owners had cut all the trees down
. I contacted the people I have given oak trees to and they had all lost them or moved and no longer had access to them.
One of the guys in fish club (Ken) had a huge collection of Bromeliads. He had all sorts and used to trade them with other people. He sold his business where he kept his plants and before he could get the plants out, the new owner took a bulldozer through the shade houses and destroyed everything.
My own mother is responsible for making a species of native plant extinct. Many years ago I found a native plant in a bush block that as about to be cleared. The entire area has been cleared and turned into houses, roads and shops. I got two plants from that block and kept them for years. They never flowered but grew every winter and spring, then went dormant over summer. They were healthy and had big nodules at the base of the roots for water and nutrient storage. The WA Herbarium wanted the plants because they were new to science. I said I will give you some seeds but your not getting the plants. In the summer of 2015 I got seeds after the plants flowered. I had them in my room and was going to give them to the herbarium when I was thrown out of my home by the courts. Shortly after that my mother and sister went through my stuff and threw everything out, including the seeds and adult plants. The herbarium never got their seeds and I lost the only two plants of that kind anywhere in the world. My mother said she threw out the dead plants and any pots that didn't have plants in them. I said 90% of the plants I had were bulbs that died back over summer, and she knew that because she used to comment on them every spring and summer. I got a few issues about that.
The moral to collecting and breeding rare or even common things, is don't have all your eggs in one basket. Share them out and make sure you can get some back if something bad happens. You never know when some moron is going to do something stupid and potentially wipe out a species.