Although I have some of the books discussed in this thread, I don't agree with the hype over a "first appearance" in an ad. While it's an interesting anecdote from a historical standpoint, I don't think it should have a significant impact on the value of a book, and it certainly shouldn't make a book more valuable than the book in which the character formally debuts.
I recognize, of course, that my opinion is contradicted by the prices we are now seeing on some of these books with "first appearance" ads. It will be interesting to see if these prices hold up over time in our CGC cover-centric world.