I found this helpful tutorial about integrating paypal buynow button with wordpress using custom post type.
<form target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="business" value="example@example.com">
<input type="hidden" name="lc" value="GB">
<input type="hidden" name="item_name" value="<?php $meta_values = get_post_meta($post_id, 'product-name'); ?>">
<input type="hidden" name="item_number" value="<?php $meta_values = get_post_meta($post_id, 'product-reference'); ?>">
<input type="hidden" name="amount" value="<?php $meta_values = get_post_meta($post_id, 'product-value'); ?>">
<input type="hidden" name="currency_code" value="GBP">
<input type="hidden" name="button_subtype" value="products">
<input type="hidden" name="no_note" value="0">
<input type="hidden" name="add" value="1">
<input type="hidden" name="bn" value="PP- ShopCartBF:btn_cart_LG.gif:NonHostedGuest">
<input type="image" src="https://www.paypalobjects.com/en_GB/i/btn/btn_cart_LG.gif" border="0" name="submit" alt="PayPal — The safer, easier way to pay online.">
<img alt="" border="0" src="https://www.paypalobjects.com/en_GB/i/scr/pixel.gif" width="1" height="1">
</form>
But I am wondering if there is an option to add a redirect for downloading the product/file after payment is completed? I wanna build a wordpress digital sale shop the simplest way without relying on plugins. However, is this method secured at all or will I be better off using an SSL/secured connection? Any help will be appreciated.