Op answered but I still don't understand

they want to buy dirty bitcoin which they think are sold at a cheaper price just to exchange back to their fiat money? it doesn't make sense if we follow op's logic.

if by op's logic, dirty bitcoin is sold cheaper than those not previously linked to illicit acts then you are also not going to make any profit because you are also going to have to sell at a cheaper price, so I do not understand the thought process behind this question