If you are on a dark market and your chat and purchases aren't encrypted with both you and your recipient's keys, then you are in the open.

You see people get easily tricked into bypassing all the provided anonymity tools... like a fake FBI seller will say, don't encrypt the messages to me twice, it is too much of a pain.....He's tricking you not to encrypt it with your key, only with his,so that anyone who he shared his key with can eaves drop.