If you are just asking why there might be one or more sets for sale, the answer is pretty straight forward. People constantly buy audio gear, try it out, and move on. Maybe they didn't love it, but maybe they just want to try something new, or move to a new space and need a different setup, or need to downgrade for financial reasons, or... the possibilities are endless.
I have encountered people who seem to think along the lines of "if a product is so good, why would anyone be selling it on the used market?" Which seems like a weird question. No matter how nice the speaker, or amplifier, or turntable, it is virtually guaranteed that we will find a used example out there at some point. For all sorts of reasons. It says nothing about the quality of the actual product. Same thing with cars or pretty much anything else you can think of.