The other person sends you an invoice or request payment.
Now you have an offer to sell. Be sure to identify what you are buying.

You send payment. Acceptance of the offer, for the payment (consideration).

PayPal should give you buyer protection.
I just bought a stem on CL, 50 miles away, gave $10 for him to ship.
Not sure if he did or not, never got it, opened a dispute and a claim.
He never tracked, so he'll either refund or be done with PayPal.
My guess is he didn't send it, or packed it so haphazard it's gone.