What is the scam here?

Got a message on my marketplace ad almost as soon as I posted it, had no questions about the item but immediately wants to send a FedEx man to collect the item and pay cash.

Screams scam to me, so obviously won't be going ahead with it.

But I am curious, what exaclty is their plan if I did? I'm not handing over the goods unless I get paid for it in cash, so I guess their only plan is to rob me?

https://preview.redd.it/ii7choy6k2aa1.png?width=490&format=png&auto=webp&v=enabled&s=faf3829d880a596b5797ee22daf278a53e406266