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, firstname.lastname@example.org and email@example.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="selle