My guess would be most people don't look past the FIRST page of cards.  Regardless, if they have put in a specific search, the card bought might be on page 1 or page 14 if it's what they want.  Just sold one yesterday where they buyer had put "blank inside" as their search.  Unusual, because most purchases, I think, have messages inside.
 

You really can't look at it that way...
 
First of all, the reason GCU has sub-categories within sub-categories is to help narrow those pages down for shoppers.
 
Secondly, if you look at the emails when you receive a sale notification, you'll see that 90% of the time, sales (at least mine are) are made from specific searches or found browsing very specific categories.  
 
So, don't worry about how many pages there are, like Rebecca, there are the occasional shopper who will look until they find what they want, regardless of the number of pages and others will continue to narrow their search through category links and/or using the Search Within a Category Feature to reduce the number of pages...that's why titles and keywords are so important.
 
Just my opinion though  

My best selling Yule/Christmas card last year comes up below another of my cards when you search for Yule cards, so I'm not sure how that happened - possibly people were searching for that specific image - although you'd think then that it would be higher than my other card when you search Yule cards by popularity?  I don't think many buyers go through people's shops other than to look at the recommended cards, as you say some of the bigger sellers have thousands of cards, I think they must search by keyword.
 
Gillean

When looking for cards, or other things online, I don't look at more than 10 pages.

If I'm looking for a special card, I can spend an hour or more! I love having the choices!