Was it an ESUS tracking number @pasten_94 ?
eBay Standard Envelope (ESE) is a metered mail shipment so it does not receive traditional tracking scans like a package would, and the clerk at the PO counter cannot scan it or track it. The envelope is scanned by sorting machinery at distribution centers along the way.
With ESE, a 'delivered' scan generally occurs at the last USPS sort facility before it gets to the buyer, so typically the buyer will receive their order a few days after the delivered scan.
If you just saw a delivery scan today then I suggest giving a few more days for the envelope to turn up.
You can sign up for USPS Informed Delivery. You get an email every morning with images of letter mail you'll be receiving that day (ESE should be pictured with the letter mail), and the email will also show a list of package tracking numbers arriving that day.
https://www.usps.com/manage/informed-delivery.htm
When an item is pictured on your informed delivery email, but that item isn't delivered, you can log in to your account at usps.com and report that you didn't receive it. That alerts the PO and they will alert your carrier that a mail piece was missing from your delivery.
