@StampSlacker: I deliberately left the seller's name out because I wanted this discussion to focus on how
ebay handles suspected counterfeit stamps and feedback, rather than turn it into a public accusation against a specific seller. If you'd like to know which seller it was, feel free to send me a private message.
@eyeonwall: Fair questions, and you caught an error in how I described the timeline. I did not discover the problem and then go back for seconds.
Purchase #1 was a block of four for under $10. Only a few hours later, before receiving or inspecting it, I placed Purchase #2 for three booklets. For the larger purchase, I had actually intended to buy sealed booklets from a different seller so I could be confident they were genuine, but I mistakenly ordered from the same seller again.
I received and inspected Purchase #1 afterward, found the problems, and later found similar problems when Purchase #2 arrived. I've corrected the timeline in my original post.
As for needing more stamps, I build multiple topical collections, so I often need more than one example of the same stamp. For stamps I didn't already have, I also compared against canceled examples shown on USPS cancellation pages.