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.
I already mention it above! it's funny how you didn't read my full post and only quote few part of my post.
Just use decentralized exchange e.g. Bisq to skip that and use reputable mixer e.g. Chipmixer.
Other mixers usually charge few % of fees and some of them ask for high minimum amount to mix the coin, but Chipmixer only ask you minimum of 0.001 BTC or $17 right now, it's really cheap.
Minimum amount is 0.001 BTC with 0.001 BTC increments.
Also Chipmixer doesn't ask you fees if you want to mix your coins, so there's no such "mixing is expensive" in Chipmixer
[Tutorial] How To Mix bitcoin free . Chipmixer's volume is high, so you wouldn't need to worry about it.