Hi,
I had never heard of Mercari before your post. I went and looked. Some of the pins look great, and prices seems spot on or even a little expensive. Other sales look inexpensive but could contain scrappers.
Seems to me, that you will run into the same issues as you do on any site like this. There will be great deals on authentic items, and there will be great deals on scrappers (meaning not a great deal at all). Part of the problem is that a lot of people don't even realize they are selling a scrapper. When I see the description saying things like - "all pins were only traded at a Park from a Cast Member" - it clues me in that the person either doesn't realize that scrappers more than often actually come off a cast member; the seller is either a new person to the hobby, and lulled into thinking that a park trade is safe, and they really don't know that they have fakes, or they suspect/know they have scrappers and are hoping it sounds good to a new hobbyist and sell the fakes as "park traded real pins".
I'm sorry I can't help you more than that, and you probably know all that stuff I just said...hopefully someone else can give you more info!
In the mean-time, I am going to go look at some more of those listings.