If they don't ship it they don't get paid. Every transaction has access to customer service if you need it and sales are monitored. You receive an email once your package is delivered. Once u open and inspect you submit a reply to email stating that you received it and its as advertised. If there's an issue you state what problem is and from there the seller will either fix the issue or you will return it and they will not be paid. (Free returns for damaged or not as described items). If something doesn't fit or you don't like it you're out of luck. (This is repeated before you confirm each purchase)

You can cancel the item and get full money back or reach out to mercari (takes awhile)