It can take extra time to cash the payment check depending on the source. Even the postal clerks don't always have enough cash in their drawer to cover postal money orders, and out of state checks can also take extra time to clear.
It can take extra time to transport depending on the distance involved, shipping method and the particular carrier.
The delivery service can also be the cause of a delay if they're not on schedule.
So can receiving FFL's if they are not available to receive a delivery.
Sometimes the FFL must pick the package up at the post office due to prior arrangement with the letter carrier.
Since packages insured by the USPS for over $200 require a signature, the FFL needs to be open to sign or they must pick it up themselves at the P.O..
Not every package has a tracking number. Postal insurance provides a form of tracking packages but a USPS delivery confirmation number isn't necessarily provided by the seller for free unless he volunteers to purchase it.
UPS doesn't transport or deliver packages on weekends, which are not considered to be UPS business days.
It's not uncommon for it to take 10 days to receive a gun after the seller receives payment.
Maybe the seller needed to order a shipping box that will fit a long gun which could take 2-3 days to receive after ordering.
If buying a gun from an auction house after a large auction it can take 20 - 30 days or more to receive an item after payment is made because they can have a lot of guns to pack and ship.
It's just like that Tom Petty song, the waaaaiting is the harrrr-dest paaart!