It's a tough situation, but perhaps Bitcoin could implement some kind of tracking system to prevent tainted coins from being used in transactions. This could help protect both buyers and sellers
Bitcoin is decentralized, if you suggest to implement such kind system it means Bitcoin no longer decentralized. You don't need to create a tracking system because Bitcoin is using public blockchain and anyone can see any transactions, address and the balance.
I guess the lesson here is to always do your due diligence before accepting Bitcoin as payment. You wouldn't want to unknowingly get mixed up with any dirty coins.
Let's say you're asking everyone about the original coins they got, do you think a criminal will answer if the coins got from hacking a centralized exchange?