I've been selling online since 2007, I've sold on ebay, etsy, and had my own dot com. Hundreds of thousands of sales..never have run into such difficult customers until Mercari. So, I have a few thoughts on what to expect, especially if you're new to online selling.
First off, unless you are selling BRAND NEW..always expect for the buyer to not leave positive feedback. I've noticed the only time I get negative feedback is when the item is sold as USED. Be prepared that no matter if you consider yourself a nit pick perfectionist such as myself, there's a chance your perfectionism isn't up to someone else's standards.
An example I can provide is, I sold a relatively new-ish pair of Torrid active leggings. The buyer left 3 stars, without an explanation and marked it off as "item cleanliness." What in the world??? I was racking my brain trying to figure out what went wrong..as they provided no additional feedback. I mean this pair of leggings practically looked brand new..I washed them and lint rolled..Then it dawned on me to look through this person's feedback to see if they left any comments. Sure enough, they mentioned to another seller how their items smelled too much like laundry detergent..So there it was..it's because they thought I used too much detergent..which funny enough I actually don't even use the suggested amount of detergent because I'm very sensitive to strong fragrance. Top tip to new sellers, make sure you wash pre-owned items with just water or a fragrance free detergent.
Also beware of customers paying no mind to your description or pictures too. I got 2 stars because a shirt where I clearly listed the measurements on the pictures & in the description said that I listed the size incorrectly. Buyer said it fit like a little boys shirt, when it was my husband's shirt and it fit him perfectly..What she should have wrote is that she ordered the wrong size and/or didn't thoroughly read the item description.
There's a few more instances that happened way before these two that caused me to put less effort into packing the little extras. No more hand written thank you notes, and no freebie stickers. I was also giving out my business card for my etsy store, and I quickly realized there's a chance for retaliation from unhappy customers so I stopped doing that months ago. Sorry:( It is true one bad apple spoils the bunch. I still do all the little extras for my etsy store though..