You see these advertised from time to time. Some collectors just don't know what they are.
Postal cards were first printed in 1873. The early postal cards were cut apart into individual cards. In the early 1900s, businesses began to use postal cards for advertising. Because the cards were cut and separated, that required the printer to run each card through the printing press individually.
Since 1875, the USPOD started issuing some postal cards in full sheets. This allowed large businesses to print whole sheets of cards at one time with their advertising. This saved a lot of time and labor.
Sometimes you'll see copies of UX27 or UX38 with the vignette in the lower left corner on the front of the card. The seller calls these "error" cards since the vignette isn't in the upper right hand corner. Or sometimes you'll encounter a pair or block of these cards which are uncut. I think some of these sellers are just ignorant of what the card really is. But I think too that some sellers are trying to pull a fast one by selling an "error" so cheap to an unsuspecting collector.
Mint sheets of these cards are still available. You'll find them from time to time in dealer stocks. With a pair of scissors, you can have a ball and create all kinds of odd shaped "error" cards.
These cards are not errors. Most are just philatelic inspirations. They are just curiosities with no premium monetary value. If you buy one, I wouldn't pay more than a few cents or perhaps a $1 for it if it was something really unusual.
However, not all postal cards were available in full sheets. There are legitimate miscut freaks of some postal cards. Those cards do carry a premium value. When in doubt, consult the Scott catalog and check to see if the card you're considering buying was available in full sheets or not.