Simple Delivery parcel hasn't arrived, no proof it was sent

Purchased an item from a private seller, who claims to have posted it on the 24th. Tracking still says "We're expecting it", and even with a shoddy outfit like Evri I'd have expected some movement by now. Just wondering what happens now, since there doesn't seem to be any proof that the seller has even sent me anything? The delivery date I was given is tomorrow.