I agree with classic_paper, the eBay
lot noted in the original post has a very clear and direct description that he is selling a single stamp broken from a certified block. Very common practice.
The reverse printings did not come to prominence until the last 20 years when they gradually became Scott-listed. Most collectors (including myself) have never bothered to examine their holdings closely for the printing sequence. Most who merely space-fill preprinted albums would never bother either. IMO, I would not pay any premium for reverse printings until I made a fairly thorough canvas of dealers/collectors who do not differentiate their stock. I am convinced most can be found for the "regular"price with a little knowledge and effort.