Mercari doesn't use paypal. They hold the money, then release it to the seller once the transaction is complete.If your payment came through PP, you should win the case automatically.
In fact, there was a recent post where a seller sold an item on ebay in April. 9 months later, the buyer opened a Paypal dispute claiming item wasn't as described but requested what sounds like a partial refund. Paypal automatically closed the dispute (after notifying the seller) because it was past the 180 time frame to open disputes.
SNAD from April 2019 recieved today