Even for 2- or 3-day priority mail, 8-21 is the new normal. Airplanes aren't flying as much, so the post office is shipping mail by truck. Coast-to-coast mail is the worst - I received a package yesterday that was mailed from the Seattle area on June 17.

The bigger question for an Ebay transaction is "when was it shipped?" - which should be part of your sales record. "When will it arrive?" is something neither Ebay nor the post office can predict right now.