Er... actually many of them are just spare parts with few bytes written, Non-OEM parts or OEM parts you name it, and sniping those with fraction of original MSRP is the most amazing thing you can do on eBay.
I grew up in one of the Eastern Europe countries, where the Russian invaders installed communist governments.
During those times, it was forbidden for individuals to buy or sell most useful things.
So there was an underground market for all the things that you could not buy from a legit shop.
Besides things that were smuggled over the borders, one of the most important sources for the merchandise were the state-owned companies. In those companies, periodically someone made lists with various spare parts or devices that should be scrapped, supposedly because they were defective or obsolete.
The parts or devices that had to be scrapped were supposed to be destroyed, but usually they were salvaged and sold on the black market. Of course most of them had no defects and were singled out for scrapping so that whoever put them on the list would take a share from the profit made by selling them.
There are some times good reasons to dispose of SSDs which are not worn out. For example some times ago I decided to use only NVME SSDs in all computers, because that would simplify some custom software that I use, and at that time I had some large SATA SSDs that were still good for many years of use. I gave all of them to various friends or acquaintances.
Something like that can explain some of the good SSDs that can be found on eBay, but I suspect that many of them might have an origin like the devices from the scrapping lists of my childhood.
Nonetheless, if I had the occasion to buy such a cheap good SSD, obviously I would buy it, no questions asked