Is this a Payments Standard Setup? If so on the confirm page in the form that sends the users off to PayPal there are two elements that handle the page the users are returned to:
<input type="hidden" name="cancel_return" id="cancel_return" value="" />
<input type="hidden" name="return" id="return" value="" />
You would need to make sure that you have the values specified here for the pages you would like the user returned to. You can also make use of the auto return feature and make sure each seller has the unique auto return URL to get to.