It’s quite hard to make up a compelling business case here.
Using a desktop solution for selling is quite niche, even more so if you are on a Mac. It barely works for us, despite the number of eBay sellers, which is probably a magnitude larger than Mercari’s.
It might be probably quite hard to get though to the right people at Mercari, and convince them to fund development and maintenance of a Mac app, given the low numbers of expected users.