Isn’t that like buying second hand crash helmets and remould tyres?
No, it would be like buying tyres or a crash helmet off ebay that stated condition as ‘very good’.
There are risks buying anything anywhere. If you dont understand the risks you go to a shop to buy it new, and pay for the privilege of someone taking (some) of the risk away. There are still always risks, and if you dont know what the risks are buying s/h then you dont know what they are buying new. Someone could have dropped the helmet from a height causing non-visible damage, you dont know, you just hope, the chances compared to a ‘s/h new unused’ one on ebay are a lot less and therefore you pay a lot more for it.
To compare fairly, its like buying a burger off someone who has just walked out of macdonalds, you dont know if they have spat in it, but equally you dont know if the guy making it has spat in it either, but you would hope they have systems in place to prevent it.
In the case of a life jacket you can check either buying s/h or new, you inflate it, like the OP did.