Assuming that it is a scam (as, of course, it
could be genuine), I suspect that you might be asked to pay a fee to cover the courier's services, with the promise that the same amount will be added onto the cash that you'll receive.
You'll then get a fake InPost invoice, with the money that you pay actually going to the scammer. Then nobody will turn up to pay you any cash (or to collect the item that you're selling).
That's based upon reading the "Facebook Marketplace Courier Scams" section of this page:
https://www.parcel2go.com/content-hub/parcel-delivery-scams