To Certify Or Not To Certify - And Is There A Cheaper Solution?

Even that has not always been true. Even very active dealers can miss good items, and have them "laying around". I happen to know that lot 6116 of the upcoming Kelleher CZ auction sat in an envelope in a box on a dealers shelf among dozens of other examples of #20 (and thousands of other CZ stamps) for 3 decades, and then the box sat for 2 decades more after being sold. It was only rediscovered when a third dealer bought it as part of an estate sale and had the whole box cataloged; my understanding is that that block had been known to exist but was considered "missing" all those years.

What part of "I can see where..." excludes that possibility? Jeez Loweez.

The US 1c 1851-57 issues have been full of misidentifications and continue to be misidentified by collectors and dealers. Without doing plating of a stamp stuck in the album space for a #9, it will be indeed a #9 in the great majority of cases but better types still pop up on occasion. It is not an everyday occurrence. Or are you implying that ignored "finds" are everyday occurences?

Forgeries are known of the Canal Zone typeset surcharges and before the Plass/Brewster/Salz book was issued, only a relatively few people knew what the genuine and genuine errors looked like. With a bunch of copies on hand, it could be easy for people of a skeptical mindset to assume they had fakes. It could also be too easy to set them aside and think "I'll get to somebody to check them when I get around to it" or to be too cheap to even send one in for expertising. Just speculation on my part, but I've seen this happen before myself, even offering to send an item in for a friend with the deal of getting repaid only if it came back genuine. No dice.