Buying a used cartridge depends on the cartridge in question. If it's a very good cartridge going for much less than the rebuild price, then it might be a good bargain.
I would not buy a used cartridge from someone I don't know / trust unless it was with my understanding that the cartridge would need to be evaluated and likely rebuilt before it ever touched the surface of any of my vinyl. (First stop before play is Soundsmith for an evaluation.) It's not a trust issue. The seller might not know of wear or damage and selling a damaged cartridge in good faith. Or they might know and are unwilling to disclose wear or damage information unless specifically asked.
But it's not touching my records before it undergoes expert evaluation.
I would not buy a used cartridge from someone I don't know / trust unless it was with my understanding that the cartridge would need to be evaluated and likely rebuilt before it ever touched the surface of any of my vinyl. (First stop before play is Soundsmith for an evaluation.) It's not a trust issue. The seller might not know of wear or damage and selling a damaged cartridge in good faith. Or they might know and are unwilling to disclose wear or damage information unless specifically asked.
But it's not touching my records before it undergoes expert evaluation.