How do I get an options set of questions for my website?

I'm using BigNoseBird - but can switch to something else if required.
What I want is a button or radio button that when checked opens a
series of additional questions.

Its better to give an example thna try and describe it - people want
to say what tools they want to sell, or what they want to buy.

They start by entering their name, address, email, telephone number,
then have 2 buttons to choose from - "I want to buy" and "I want to
sell"

Depending on which they select, a further set of questions will
display that also need processing in the same form to email script.

If they choose "I want to buy" then 6 more questions should display 
q1) 3 options as check boxes
q2) text box
q3) text box
q4) text box
q5) text box
q6) 3 options as check boxes

If they choose "I want to sell" then 5 more questions should display 
q1) text box
q2) text box
q3) text box
q4) 3 options as check boxes
q5) text box

Client actually wants these additionally questions in a popup, I don't
see how that will work but I can see it working as a drop down
triggered by a javascript or something, in fact there is a similar
script running at this page:
http://www.myskillstest.com/?page=testdemo&module=Spreadsheets click
on test me and you will see the radio button for test me triggers the
addition and display of more questions.

Please help - or for $25 anyone want to actually do this? I can create
the fields and form butons, etc - its just the drop down and tidying
it all up after thats an issue for me.

Thanks in advance,

Scott