... buying them thru the Philatelic Sales catalog. They didn't go over well, apparently ...
A really good example of really insular thinking.
The LAST DAY OF SALE "event" only "happened" back at the philatelic sales office / fulfillment center (or whatever it was called twenty years ago).
Out in the field, while some postmasters would have dutifully shipped back sealed unsold stock, there is no way that every postmaster went thru every window clerk's stock and pulled partial sheets (for example); therefor, the stamps would have continued to be "on sale" at post offices all over the country after the "last day of sale".
And, since the stamps were not demonetized - and were still good for postage - the "last day of sale" was not something actually important like, say, a "last day of (lawful) use" ... you find a stamp of the Shah of Iran GPU (Genuinely Postally Used) in Iran after he was deposed and, yeah, you've got something.
This kind of insular thinking is a plague on humankind; Vietnam was a while ago, but some of you younguns might remember Netflix deciding to dump their so-last-year DVDs-by-mail business into a separate any-name-but-ours brand ... or, then, maybe, you don't.
/s/ ikeyPikey (who would love to re-read & re-savor Barabra Tuchman's The March of Folly