You can do this with a simple javascript trick.

Step 1. Remove the <Form> </Form> tags from your PayPal "Buy Now" code.

Step 2. Add 
          onClick="this.form.action='https://www.paypal.com/cgi-bin/webscr';   

                      this.form.submit();" 

      to the buy now input button.

Step 3. Paste what is left into the html module

 

What it does is change the action for the Form that is already embedded into the DotNetNuke default.aspx page.


Here is an an example:

 

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<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="payments@snapsis.com">
<input type="hidden" name="item_name" value="Buy John a Beer">
<input type="hidden" name="item_number" value="Beer">
<input type="hidden" name="amount" value="1.00">
<input type="hidden" name="no_shipping" value="2">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="bn" value="PP-BuyNowBF">
<input type="image"

   onClick="this.form.action='https://www.paypal.com/cgi-bin/webscr';this.form.submit();"

src="https://www.paypal.com/en_US/i/btn/x-click-but23.gif" border="0" name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
<img alt="" border="0" src="https://www.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">

 

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You can find more advanced ideas in this thread on the asp.net forums.