What would this scam be?

[Realmayowitheggs]

I am selling a few bits in marketplace. I got the following which is clearly a scam but I am trying to work out how? Money laundering with fake notes?

Ok the price suits me , Ok I buy. However, I will make your payment via UPS EXPRESS delivery in an envelope as soon as you receive the money, I will send TNT to your home to collect it.

Fake cash?

There will be no payment or they will pay by cheque which won't clear or will be fraudulent so you lose your money that way

OR they will say the delivery driver will have the payment except he won't and sometimes people hand over the goods and say hang on where's my money? He says I dunno and leaves with your goods most people are too polite to argue with the collection people because they are "just doing there job"

Thanks. I had forgotten about cheques. Might be that then.