Here’s what consumers have asked with answers from Mercari staff and previous consumers.
No this is not normal but I'm not surprised they would do something like this
You offer a price and then pay. The offer is sent to the seller. If they accept then you will receive a message in your notifications. Your payment is processed so no need for you to do anything else. If they don't accept you will be notified and no payment is made. Its all done through their app and pretty simple
I don't know how they do it but they deff monitor conversations btwn seller and buyer and make sure that if seller has not shipped product out within the 3 days allotted time that they are for sure sending them a stern reminder to ship out on time. Seller is not paid until buyer receives and inspects purchase and submits approval.