I use eBay for research--buying books, magazines, and other items relevant to my writing subjects, after which I resell them (almost always at a loss or break-even point). I also wrote a book about using eBay successfully, titled The eBay Survival Guide. So I have spent hundreds of hours at the site and have become sensitive to the ebb and flow of certain kinds of merchandise.
In recent years I've been able to tie part of that ebb and flow to the appearance of books. For example, early in 2007 I saw a definite increase in the the number of Crosley items on eBay. That includes cars and auto-related items, appliances, radios, books, magazines, etc. The number dropped off by about 20 percent as 2007 closed.
The increase was probably due to the publication of CROSLEY: Two Brothers and a Business Empire that Transformed the Nation, which came out in November, 2006. The decrease was the result of the market having been largely satisfied by more Crosley-aware sellers putting more Crosley items up for sale.
There was also a price spike in the first half of 2007. But I observed a definite trend toward lower prices as the number of Crosley items offered fell. The Simplicity of Radio, by Powel Crosley, Jr., for example, now sells for a quarter of what it brought in a couple of years ago. The price decrease is most likely caused by the market-saturation phenomenon behind the drop in the number of Crosley items on eBay.
A couple years ago I watched another book, titled Barclay Toys, Transports & Cars, 1932-1971 bring hundreds of items to eBay--Barclay slush-mold toy cars and the like. The numbers for Barclay items are still up, and so are the prices, though not dramatically so.
I'll be following this phenomenon as other books on collectibles and history (including two of mine coming out in 2008) hit the market. This could lead to a new marketing strategy for eBayers: if you have a large collection of a specific kind of item, try to get a book for collectors on the market, to increase awareness and drive up demand. And if you're going to write a book on collectibles, or about a famous person or historical events, think about stocking up on related collectibles, so you can cash in after your book comes out.