I started selling on the Mercari app quite some time ago, upon selling on there I only had two problems.
One was a buyer claimed a game in a bundle did not work, however I even had photos up that the game did work. So if you are selling on Mercari and you're selling electronics, try to upload photos proving the items work. The buyer continuously insisted the game wasn't working and Mercari told them in order to get a full refund they would have to send the entire bundle back, which they declined, however I decided to give them a partial refund. Even doing that the buyer left negative feedback which all sellers will agree, sucks.
The second problem I've had while selling on Mercari was a seller received an item broken. I carefully packaged the item and covered the package in "fragile" stamps. I do believe the post office was too rough with the package causing the item to get crushed. Both me and the buyer messaged Mercari and Mercari refunded the buyer, understandable but they also say no refunds are given until the seller receives the item back, which I did not. They did tell me to open a case through USPS to get a refund however the shipping Mercari offered at the time did not come insured. So because of USPS being rough with the package and Mercari not offering insurance through shipping I lost money.
Both problems were minor and I've never had any other problems selling on Mercari although I did stop selling after too many problems with Mercari's customer service when buying items.
Now buying on Mercari is an entire different story. I've made quite a few purchases on Mercari and I've had quite a few problems with purchases on Mercari but I am just going to go over the major problems.
One was a laptop. I needed a new laptop bad because mine was overheating, causing the digitizer to go out. So I looked around for quite some time, then I found a nice HP laptop that said it was as good as new, the seller had all positive feedbacks and I commented on their listing, asking about the battery, heating, and condition of the laptop, the seller claimed it was "as good as new". Upon receiving the laptop I noticed the serial numbers were scratched off which raised a red flag for me but I plugged it in anyways and noticed it got hot, really fast. Before 30 minutes passed I had to shut it down because it was so hot. Automatically the seller started messaging me to "rate her" once you rate someone you can not open a case, before I could even reply to her message she was commenting on my listings to rate her and if I didn't I was a scammer. So I messaged her about the flaws and she started to harass me so I told her I'd message Mercari and let them handle it. This seller continuously messaged me through out the time Mercari took to "solve" the case. They asked for pictures and I sent pictures of the laptop, the condition, plus the messages she was sending me. In the end they said they couldn't "see" it overheating so they would not let me return it for a refund.
Now on the the another problem I had with Mercari, while the case for the laptop was open I purchased some Beats by Dre headphones, before purchasing I asked if they had nay flaws, the seller said no so I purchased them, When I got the package I found a note saying the cord is messed up and you have to hold it out to hear the beats.. I messaged the seller and she said that wasn't a flaw, although clearly it is. I went to a store to get a new cord, to only find out it wasn't the cord that was messed up but the actual jack. As soon as Mercari closed the laptop case I messaged them about the Beats and they replied saying I didn't message them within 3 days after receiving the headphones and they completed the order. Now I find this unfair considering they only let you message them with one problem at a time. Now I have a laptop that you can only use for around 25-30 minutes before having to let it rest and a pair of broken beats and I'm at a loss for $200+.
So with that being said in my opinion Mercari is a great app to sell on but not so much on buying expensive items, like electronics. Also if you decide to take a chance on buying something expensive be sure that you don't have a case already open or purchase expensive things back to back. After the laptop and Beats incident I no longer buy or sell on Mercari because they really do not protect the buyer like they claim to.