In other words, I want to be able to set my own price for a book,
rather than choose from prices that are listed. I also want a seller
to see a pop-up that lists what I am willing to pay. An acceptable
answer will have explicit instructions on how to do this. I will not
pay for the answer "It can't be done."

You can set your own price in Amazon Auctions.  Otherwise, I'll tell
you for free that I'm pretty sure it can't be done currently.

wurzelma,
From Amazon?s main page, click on [HELP], in the top, right corner.
Click the 2nd option [Ordering]. Click 5th option [Ordering from
Amazon Merchants]. Click 2nd option [Amazon Marketplace]. Click 6th
option [Pre-Orders]. This page will explain how to pre-order used
books, for which you can name your own price. However, this method is
only for books which are not already available used.

If the book you want is already available used, you can contact the
seller, tell them that you want to buy the book and what price you are
willing to pay. (I have sold books on Amazon and am sure most sellers
will consider any reasonable offer, especially if they are having
trouble selling the book at their price.) To do this, find the book
you want, and go to the listing of sellers. Click on the name of the
seller you wish to contact. This will bring up the seller?s page. On
the right side of the page, there is a [Related Links] section, under
which is the option [Contact this seller]. Click on it, and you can
email the seller directly.

Happy book hunting
Rachel