Mercari is a place where you can buy and sell things from other people like you. I'm assuming this is a Japanese venture because the site can be set to "United States" and "Japan" but you won't find a link to their support system. Support seems to be available in app only.
For buyers: When you find a product you like, you can buy it, assuming that the seller still has the item and the app on their phone and is paying attention. Usually after a certain amount of time if the seller hasn't shipped on contacted the buyer through the app, the transaction can be cancelled at the request of the buyer.
For sellers: You can choose up to four photos and edit them in the app. You need to add a title and description, as well as choose the shipping fee, condition the item is in, size (if applicable, and price. Whole dollars only, no change.
You get $2 for yourself and a the person you refer to Mercari (if you want to use mine it's jld475, just saying).
Once you receive the item you ordered you will have to rate your seller, and sellers will have to rate the buyers. If you are not satisfied with your order when it arrives you can figure out what to do by contacting Mercari and/or the seller. Do not, under any circumstance rate the other person if you are unsatisfied with your purchase because there's nothing that you'll be able to do to get your money back if you do that.
Payment methods: Paypal, and card. You can have funds sent to your bank through the app.
Shipping methods: Labels are offered for USPS and FedEx. Mercari recommends using USPS for everyday things and FedEx for fragile things. The shipping rates are: 0-0.5 lbs = $4,0.5 - 3 lbs = $6,3 - 10 lbs = $9, and 10 - 20 lbs = $15. You can also choose to ship the item yourself.
I hope this is helpful. I have had a pretty positive experience using the service for several months now. I've had two negative experiences but one of them had deleted the app so they never knew I had purchased anything and another lady failed to disclose to me that the eye shadow palette I had bought from her (which she said was "no brand") was actually a knock off of something else. I did report the account.