As we all know the blockchain is a public network and sometimes it is ued for buying drugs, weapons etc. For eample if I am going to sell bitcoin to someone I don't know what is the best way to stay anonymous after such transactions? So in case my buyer will use those bitcoins for buying drugs I would be able to stay away from it.
First of all no one knows that you owned the address. If it will just be traced if you used an exchange, verified yourself and so on.
In here you are worried that if you sell your bitcoin to Person A then that person used this coins to purchase drugs you will be involved now to the illegal activities. It's out of the context now so don't worry. Although it has low chance you will be interrogate, and let's say you are verified in an exchange you used to send the coins, you can give your side.
It's difficult for authorities to traced if the coins where being used to purchase drugs since exchanges have thing called "inputs" and some of them have different public addressed recorded on the network where you send the coins.
P.S Dont advice OP about mixers. He is asking about the scenarios, as if he will know that his buyers will used that coin to purchase drugs or I doubt OP will use mixers everytime he will sold some coins. That will take more fees.