Various forms of transactions were made by criminals. At present with the ease of transaction uᴉoɔʇᴉq is getting popularized among them. So as a part uᴉoɔʇᴉq is also used in illegal transactions.
Yeah, mainly for transferring funds without being noticed by the authorities.. But I think if they are about to pay some illegal goods that they bought, fiat would still be the best and hand to hand, not thru fund transfer...