Thank you very much, Ben.
Well, after searching and testing, I found out that the code in Jon's project files is still useful.
For those of you who got the same problem as mine, here is the solution:
1. Create 2 dummy accounts: one for selling and one for buying as Jon said. (For example, seller@gmail.com and buyer@gmail.com)
2. Modify your shopping cart so that is looks like the code below: (Supposed that your cart's items stored in $items variable)
<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="business" value="seller@gmail.com">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="upload" value="1">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="lc" value="US">
<?php for ($i = 0; $i < count ($items); $i++) : ?>
// Display $item, not shown here
<input type="hidden" name="item_name_<?php echo $i; ?>" value="<?php echo $items[i]['name']; ?>">
<input type="hidden" name="quantity_<?php echo $i; ?>" value="<?php echo $items[i]['quantity']; ?>">
<input type="hidden" name="amount_<?php echo $i; ?>" value="<?php echo $items[i]['amount']; ?>">
<?php endfor; ?>
<input type="image" name="submit" src="https://www.paypal.com/en_US/i/btn/btn_buynow_LG.gif" alt=" pay online with PayPal ">
</form>
Well, it is exactly as what Jon guided us. We just need to ignore the part creating the code for Buy now button because as Ben said, PayPal has changed their forms