Yep, what he said is how the sting operations work: undercover cops will buy btc, and casually mention that they plan to buy drugs, a whore, aor anything else illegal.  You might naturally shrug it off and say that it is none of your damn business what they spend the money/btc on.  But as soon as they tall you, BAM you are now a party to criminal behavior.  To avoid this, if anyone even HINTS at illegal activity during a trade, jsut walk away from the trade.  You can politely say "sorry, but I can't trade with you now that you have told me that," or something similar.  But watch your ass, they are trying real hard to put btc on ice.