You may be complicating matters by assuming you need to select  a button.
Buttons simply call another page. Click the button and look at your
address bar to see the URL of this page. Sometimes, a selection made on
the previous form, i.e. your list  box, will appear as a value in the
URL. All you need do is call this URL page  with the value you want from
the list box. Here is an example.

Dim sURL as String

sURL =
"www.overstock.com/.../d2.cgi
&N=0&keywords=" & sISBN

I had to write a program sending ISBN information as we work with this
company. Take note on how I send  a string  variable to the web  page. The
previous page on their sight has a text box  and search button, but that
button leads to the above URL. I simply bypass the search page all
together.