I have bought many cartridge used over the years (mainly fitted to Rega decks that I have purchased), and on only a couple of occasions can I say that I had a worn out cartridge. People change cartridge frequently looking for a better sound, so often the cartridge has not had enough use to wear it out, though I would never spend a lot of money on one, and when you get it, only try it out on non-precious vinyl first before committing it to your collection. You will soon know if the stylus is worn, it is unlikely to track very well, can sound very rough (though this can be like this if the cartridge if a low cost one anyway), and lack detail in the high frequencies because the stylus cannot track into those undulations any more.
Some sellers show magnified images of the stylus showing how unworn the stylus is; If the image is genuine then this would be a good way of knowing, but you cannot trust this, and must go on the reputation of the seller more than what is shown or written in a listing. If the seller sells hi-fi, look on their feedback to see if they have sold many cartridges and had good feedback for them, indicating that they have morals and won't sell worn ones.
You can get a really good cartridge for much less than a new one of you are careful.
I hope this helps you.
Rge Spares by SoundsSupreme
Some sellers show magnified images of the stylus showing how unworn the stylus is; If the image is genuine then this would be a good way of knowing, but you cannot trust this, and must go on the reputation of the seller more than what is shown or written in a listing. If the seller sells hi-fi, look on their feedback to see if they have sold many cartridges and had good feedback for them, indicating that they have morals and won't sell worn ones.
You can get a really good cartridge for much less than a new one of you are careful.
I hope this helps you.
Rge Spares by SoundsSupreme