If you are creating buy now button by hand and if you want to give customers an option when they are buying a product with PHP-eSeller, you can easily do this using a drop down list.
An example might be :
<form action="http://www.yourservername/eseller/ipn/process.php" method="post">
<option value="">Select a product</option>
<option value="item_a">Small $1.00</option>
<option value="item_b">Medium sized file $2.00</option>
<option value="item_c">Large file $3.00</option>
<input type="image" src="https://www.paypal.com/en_US/i/btn/x-click-but23.gif" border="0" name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
You will need to create three products with item_number of item_a, item_b and item_c. The customer then selects which one they require and click the buy now button which takes them to PayPal.