eBay requires sellers to have a valid acceptance scan from the shipping carrier within the stated handling time in order for a shipment to be considered "on time". If sellers don't have this scan, the shipment may be considered late and counted against the seller's metrics, which can negatively impact the sellers account in several ways.
USPS and other carriers often require sellers with a large number of outgoing packages to use a scan form or manifest of some kind to speed up the process of picking up and scanning packages.
According to USPS:
The SCAN Form features a master barcode that represents all of the packages in a shipment; it is scanned when the shipment is received by the U.S. Postal Service®. This single scan enters all of the associated packages into the Postal Service USPS Tracking® database as “Shipment Accepted” and allows both the sender and the recipient to see when the package entered the mailstream.
Some eBay sellers have been reporting problems with printing USPS SCAN forms for postage purchased on eBay for over a month now with very little help from eBay on the matter.
The problem appears to be mostly related to labels printed with eBay's Bulk Shipping option. Sellers are able to print the shipping labels without a problem, but eBay fails to generate a corresponding SCAN form to go with those labels.
Many of these sellers are now stuck between a rock and a hard place. The carriers won't continue to pick up and scan hundreds of individual packages and waiting in line at the post office to get scans for each package is not a viable alternative either.
If sellers can't get SCAN forms for the postage they purchase on eBay, their only other option may be to purchase postage elsewhere through companies like ShipStation, Shippo, or PirateShip that can reliably provide this functionality.
At eBay Open in 2019, then CEO Devin Wenig reported eBay was the second largest USPS retail customer representing $1.5 billion in postage. It's long been speculated that eBay may have used that #2 position to negotiate discounted rates above and beyond the discounts provided to sellers, potentially representing a profit center for the company.
If that is the case, I have to wonder why eBay wouldn't be rushing to fix any technical issue that could possibly be driving sellers to purchase shipping labels anywhere else?