FWIW, some merchant which respect privacy is less likely to check whether deposited coin is "tainted" or using payment gateway which automatically check deposited coin.
My opinion is that we shouldn't even care about what they do.
When we use any type of currency, we 're just owning the currency until we exchange it to buy something.
So, for this little time, we shouldn't be feeling guilty.
For example, let's say that the European Central Bank decides to create 1 banknote worth 50 euros.
This banknote has a unique identifier and so, from the day it is produced, it is uniquely identified among all the other banknotes.
This banknote now travels around the world, changing hands, being used to buy ordinary staff (cigarettes, beverages, PC parts etc.)
After changing 50 different hands, it goes to a drug dealer for selling cocaine.
Now, it immediately goes to me.
I am then spending it to buy coca-cola from the local market and the merchant is a secret agent who identifies the banknote is used for illegal purposes.
I am prosecuted.
I 've nothing to do with any of that, I am ok, I am free.
Wrong?
You're not wrong. But would you rather spend your Bitcoin on merchant who discriminate incoming Bitcoin (usually also with little explanation/transparency) or merchant who doesn't do that?