You seem to have a fundamental misunderstanding about how forms work. Forms gather name/value pairs from the form input elements and send them to the url specified in the form tag's action attribute. The name/value pairs that you are interested in generating come from this form:

<form action="/servlet/SearchResults" method="post" name="homepageSear
+ch">
[download]

How do I know that? I clicked on View Source for abebook's homepage. So you have to send the name value pairs to the url specified for the action attribute--which you should note is a relative url. Try the following url instead of http://www.abebooks.com:

Note also that the form specifies 'post', so you need to generate a post request--not a get request.