If Id sell/buy BTC from a guy and he tells me about the crimes he is going to commit with these, I'd definitely question, why he would tell a stranger about the crimes he going to commit with these BTC and Id never do business with him. How can someone be that dumb and even more tell him - person to person - that he "will think about" buying stolen credit cards? Amazing ...
a> how do you know they didn't question. does the law require them to verbalize their concern?
b> since the 30k transaction never happened, there's no proof they were willing to complete it after hearing the Russian story.
c> if you suddenly realized you were either talking to the russian mob or talking to undercover cops without knowing for certain which, would you respond by self-righteously trying to give them a lecture on how you are better than them for not dealing with stolen credit cards, or would you brush them off with a meaningless response to their question like "I will think about it" so as not to risk insulting them or leading them to believe you're a threat?
The law does not require you to self-righteously confront potentially dangerous criminals. And, saying you will think about something that someone just asked you is not the same as proposing an idea. The former is being polite and avoiding confrontation. The latter would be an example of demonstrating "intent".