So I been using Mercari for about two months+ I've sold about 20 items and bought 3. I have not had any issues at all, each and every transaction been smooth and simple.
If you want to join Mercari I HIGHLY advice you read the rules carefully. I was surprised to see how bad the reviews are for this app, but reading them I can tell people don't read the rules carefully and expect a lot from Mercari when something doesn't go correctly. SO you are putting yourself at risk by proving your address for shipping labels and bank account to receive the funds. YOU have to know what you are getting yourself into when you start selling or buying.
When someone buys an item you are selling- you ( the seller) have three days to ship the item- which I believe is too long. When the item is delivered to the buyer THEY have three full days to accept they have received the item, rate the item, and then you the seller have to rate the buyer, THEN you get paid. I think that process is SO annoying because I have had 4 people not rate me so I have to wait three days for the system to automatically provide the funds to me. I think its so incredibly rude that people wouldn't rate the seller. So with that being said, people write these horrible reviews who don't read the rules then trash the app because something went wrong.
Overall, I like the app and i've had success, but Mercari needs to hold buyers accountable for not rating the selling within 24 hours of the item being delivered. If Mercari wants to have fasters transactions and quicker turn around with products being bought and sold they need to hold buyers and sellers accountable for shipping faster and accepting delivery faster.