On further investigation, I've discovered this is less exciting news than I thought it was. Mercari has an in-house crypto exchange service called "Mercoin" which allows Mercari users to buy and sell ETH and BTC, transferring value from both currencies to and from their Mercari account balance (which can be used to buy things from the Mercari platform).
The catch is, you can't transfer any tokens bought via Mercoin off-platform to your own wallet. It stays locked into Mercoin/Mercari. This means the only real use-case for Mercoin (that I can see) is to speculate on the value of BTC and withdraw the proceeds to buy shit with.
There's a possibility they'll open up the platform to allow external withdrawals/deposits in the future, but they're not indicating they'll do that right now so it doesn't seem wise to bank on it. On the whole, this seems pretty pointless.