Yes Ive actually owned many of the blank back cards. The stock is the same thickness and print is the same quality. I think it was a sheet made for a store display. Ive seen them a couple times in groups on eBay in the past, sort of like what you mentioned now on eBay. All Ive had were sort of badly handcut (years ago?) with the sizes varying a bit, and Ive also seen a few of the bio back cards also handcut too, so the sheets were also known with the back printed as well.
The toughest cards are the "ad back" cards, and obviously the 1927 Ruth variation, and the 1925 preproduction proof cards (see below).
example of 2 blank backs I have now
1925 preproduction proof sheet (sent to Geo Kelly)
The 1925 proof cards are also known as singles, Geo. Kelly was sent a few back in 1925 with a letter asking for his permission to be used in the set, it was a follow up letter asking again after he never replies to the first letter (he rejected the offers), it was for sale on eBay within the last couple years, along with the sheet above and a few of the preproduction proof singles.
example of the back of a single card form the 1925 preproduction proof style cards (also sent to Kelly)