How to know the buyer isn’t scamming me

Description

I am selling an item from gumtree and the buyer said he would sent the money from his PayPal directly into my bank account. The money was apparently sent this morning, but I got an email from someone called Mary saying that “This is a legit transaction that has been monitored by our fund verification team, and the payment of *I blanked out payment* is pending to be credited to your account, so we give you 100% guarantee and assurance on this transaction. All you need to do is to complete the Postage at Royal Mail Postal Office and send us the Tracking Reference Number from the receipt and we will release the pending fund to your *bank* account.” I just want to make sure this payment is legit and I will get paid as soon as I ship the item, and not that it’s fake emails