Mercari is the perfect app to sell all your things.
PRO: You can ship nationwide
Shipping nationwide allows you to reach out more people which also means there is a higher chance of selling your item. Personally, it's fun for me to see where my item is getting shipped to.
CON: You don't get your money immediately
So, this might sound confusing but I am going to try and explain the best way possible. So you list an item and someone buys it. You then have to package and ship the item through whatever mail carrier you choose. Then the customer not only has to receive the item, but the must also leave you a rating. After they leave a rating, you'll receive your money. You may be thinking, "what if they forget to rate me?" Don't worry. They have 3 days to rate you and if they don't leave a review before then, Mercari will automatically leave a 5 star rating for them. Getting your money can take anywhere from 5 days to 2 weeks. So if you are wanting money fast, Mercari isn't the way to go.
PRO: You can make offers to those who like your item
This is great because it ups the chances of you selling your item. It also allows them to view your product more. Let's say you don't have any likers on your item, you can public promote it and push it towards to top of the search results when someone searches your item.
CON: There are scammers
This is on any selling platform, but there are people that leave reviews that are absolutely fake. For example, I sold a Lululemon tank for $23. This girl already wasn't willing to be negotiable with me so I had no choice but to take the amount she offered, but her reviews was nothing but lies. She said there was snags all in the tank top and thread coming from the seam. I knew this wasn't true and I even went back to the listing of the tank to see what she was talking about, and there was no thread nor snags. She lied.
PRO: Just because you get one negative rating, doesn't mean you'll never be able to recover.
Going on from the last point, that girl left me a 3 star review, but I have 55 5 star reviews so of course, when people see my profile, I have a 5 star profile.
I've been on Mercari for 2/3 months now and I've made $400 selling 40 items. I only sell things when I have things to sell, but some people make it a full-time job.
Mercari is a great way to make a extra bit of money while still being in school and/or having a job.