@factsdontcarehave you had any problems receiving the payments after sales? I read many forums about Mecari not paying or taking forever to give sellers their money.
I have had very minor issues with Mercari (I only sell...1300 sales on US acct/71 sales on UK acct/ 2221 sales on Japanese acct). Their business model initiated as "shipping only" (same as eBay) and most of the bugs were worked out before the app deployed in the US, since it started in Japan. As with most of the selling apps, there is minimal customer service support, but the app is efficient.
Most of the problems that other Mercari users have expressed is a result of the shipping delays/problems. As with any selling system, I keep immaculate records (I keep video/screen capture for all transactions in real time), so if there is any glitch on the server end, I can provide all the proof/documentation on my side.