if someone pays you by paypal and they send a courier that they book themselves then you the seller will have no proof of the goods being delivered
so... once the buyer can confirm the item was picked up from you they can then file a claim with paypal for goods not received and paypal will refund the buyer the payment they sent you.
that is the scam ... its very simple they are hoping you will let the courier that THEY paid for pick up the goods ,that is what fucks you...
if you accept payment for goods via paypal other than gift then you as the seller are fully responsible to book and pay for the courier yourself and be able to provide proof of payment for the courier service to be able to avoid a paypal claim for goods not received , those are the paypal seller protection rules guidelines
so in short to avoid getting fucked ,if you accept paypal other than gift then always book a tracked courier to send the item and pay for the courier using paypal . simple
scammers are simply preying on people who have not read or do not understand the paypal seller protection rules.
if someone pays you by paypal and it is not gift you MUST book and pay for a TRACKED courier yourself and pay the courier by paypal to be safe ,do not use a courier unless they use a signed for delivery, drop off is no good