Lost item from Mercari

Hi all, I'm freaking out a bit, so forgive me if my post is unclear. On March 10, I ordered an item from Mercari Japan. On March 14, I was notified that it had shipped, but when I came home today, it still hadn't arrived. I contacted the seller, who told me that the tracking information (which I can't see; they didn't give me a tracking number) said it had been delivered on March 16. I know this isn't the case, because I didn't receive anything in person and nothing is in my mailbox, including any notification of a delivery box being used (my apartment has codes on the delivery boxes, and delivery drivers are supposed to leave a slip with the code in your mailbox). I've asked the seller to confirm what address the package was delivered to and if they can contact the shipping company (as I have no information about the shipping company or tracking number to contact the shipping company myself), but they've stopped answering me. I'm really not sure what to do…

by tortxrra