How many of us are mixing our Bitcoin before we send them to somebody else?
I'm sure only very few.
Don't worry about it, that's why Bitcoin mixer exist and anyone can use it to protect their privacy. You can't know the background of the buyer or seller, there's no way to verify it except you're a police that can track his criminal record.
You always need to make sure the mixer is trusted to not get you screwed, and you need many people mixing to make the system efficient, mixing is expensive, honesty is priceless. just use your coin in an honest way.