If you join the chain by accepting to pay black money to a vendor, this is decreasing your chances of potential buyers in the future. I.E. you can only sell to someone that is aware and prepared to do the same.
Basically it is a TAX dodge and WILL eventually be found out.
If you start the chain off, then you are leading yourself down a slippery slope as everything once found out can be traced to source.
If someone is asking you for a briefcase full of cash to purchase, turn on your heels and walk away.