Thanks for the idea there, I got around to trying to fix this today and I am coming up blank on where to put the nofollow to get it to parse correctly.
I'm assuming I need to edit this code:
$prod_list_contents .= ' <td align="center">' . tep_draw_button(IMAGE_BUTTON_BUY_NOW, 'cart', tep_href_link(basename($PHP_SELF), tep_get_all_get_params(array('action', 'products_id')) . 'action=buy_now&products_id=' . $listing[$x]['products_id'])) . '</td>';
However no matter were I try and put rel="nofollow" the link is not coming out correctly.
I don't really think I need to check to see if there is an action set, so I was just trying to stick the tag in there. I can't think of any reason for these buttons not to have a no follow tag, since the product image and the product name already link to the product page, and the button will always link to "add to cart" action.