As a buyer and seller on Mercari I have to say that it isn't very seller friendly. One customer tried to cancel the order even after I had shipped and added the tracking info. Mercari sends me and email requesting to cancel in which I hit the "already shipped" button which should keep the order open. The buyer then tried to do it again a few days later and I again hit the "already shipped" button. Well a few days later my order gets cancelled. I am out of my product and shipping cost and the buyer gets a free item. Also there is no way of contacting the buyer once an order is cancelled.
Also as a seller I shouldn't have to wait to receive the money from my order until after the package is delivered and the customer writes the review. On ebay if I sell something I get the money right away and if there is a problem the money can be refunded.
In the case you need to talk to customer service you can forget about it. It's almost impossible to actually get to the page where you can send them a question, it tries to answer everything with faqs which are basically useless when you have an issue.