- How will you whitewash the money? Alternatively how will you spend them on the black market? You can't buy houses, cars or stocks with black money.
- Will you get paid? - Secure anonymous payments that are guaranteed are not trivial. I don't know if there are escrow services for the black market, but this is definitely risky. We are talking about shady actors after all.
- Will you get caught? If do you will probably end up in prison.
When you take the above in to consideration I think most people would prefer $10.000 legitimate US dollars without risk to $100.000 that might end up giving you ten years behind bars.