Any business that takes cash from anonymous customers for a product that is difficult to verify has the potential to launder cash.
So, a business that trades in new cars will only be able to declare a profit based on the difference between the cost of cars purchased and the amount they sold them for.
On the other hand a massage parlour can claim to have had hundreds of customers each day - limited only by the number of rooms they have and 24 hours in a day. Impossible after the event to verify how many punters they had in, so impossible to prove that some of these were fictitious and that some of the "income" declared was dodgy money given a legitimate "history". Sunbed parlours are similar apparently. Just keep the sun beds on all day, show HMRC your electricity bill, and who's to say you never had paying customers in all day?