Horrible - I ordered shoes for almost $100, and the shoes sent to me were two sizes smaller than listed. The seller conceded the mistake and I notified Mercari right away (within the 3-day required period), providing the six photos they requested to show that the shoes were the wrong size. The burden was put on me to ship the shoes back ***within three days***, with the refund (allegedly) to be issued after the seller received the pacakge; I was not able to meet the 3-day period for reasons outside my control (including the fact that I was transitioning jobs that week) and notified Mercarci but Customer Service would not help me or give more time and acted like it was my fault and my responsibility to bend over backwards.
Whenever Amazon has made a mistake, they have refunded me immediately and usually not required a return. I am distressed and have spent hours messaging them. I am down almost $100.
The whole practice of treating this as a return is flawed; I was not trying to return an item I ordered and didn't like, I was asking for a refund for boots in my size that I was charged for but never received.