Paypal hides its Resolution Centre under Tools at the top of your account page.
You have 180 days from payment to make a claim.
You have 60 days from purchase to leave Feedback for the non-shipping seller. The most effective FB is calm and factual.
"Doohickey not recieved, refund promised but not sent" for example.
This does nothing to get your money back, but will warn future customers.