how to redirect paypal sandbox to localhost (asp.net's ) after successful transaction ? is it possible ? silly question but may be ....................

You need to include the markup in your post that you've tried. I think you need to include the return url hidden field that contains the absolute URI of your webapp's resource/page.

<form ....>
  .....
  <input type="hidden" 
         name="return" value="http://localhost/WebSite/page1.aspx" />
  ....
</form>
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