I have talked to Paypal a while ago and they came up with the following solution that works:
There is a way to replicate the functionality of a Buy Now Button in a way which will work correctly on the WordPress site.
In your WordPress page where you want the button to go, click the icon used to insert an image. Here are a few addresses of images you may find suitable:
https://www.paypal.com/en_US/i/btn/btn_buynow_SM.gif
https://www.paypal.com/en_US/i/btn/btn_buynow_LG.gif
https://www.paypal.com/en_US/i/btn/btn_buynowCC_LG.gif
https://www.paypal.com/en_US/i/btn/btn_paynow_SM.gif
https://www.paypal.com/en_US/i/btn/btn_paynow_LG.gif
https://www.paypal.com/en_US/i/btn/btn_paynowCC_LG.gif
Now, leaving your WordPress page open, open a new window, and do the following:
1. Log in to your PayPal account at https://www.paypal.com/.
2. Click "Merchant Services."
3. Click the "Buy Now Buttons" link.
4. Complete the form
5. NB: Choose not to encrypt the button.
6. Click "Create Button Now."
7. Copy the Email Link, which is in the second box.
Ignore the HTML code in the first box.
Go back to the WordPress window, and on the page editor screen click the image which you inserted previously. Make the image a web link by clicking the icon on screen, and for the URL paste in the Email Link which you just copied on PayPal.
Save the page and you should have a fully functioning Buy Now Button on your site.