You can't print a usps priority label online (paypal or usps) without automatically getting a free tracking number. [ by the way they have now changed it from "delivery confirmation" number to "tracking" or "insured tracking" number on all packages sent by aforementioned methods]
Up until a few months ago it was simply a "delivery confirmation" and although it very well might include other scans, it was not required. But now my carrier says they require scanning no later than by the time they get the package out of their car.
So, your seller must have hand delivered / paid for the package at the post office to not have a free tracking number.