when seller update the order status to complete on their site, it shows completed but the actual status is still processing. therefore that order is not updating when it is time for the seller to withdraw. pls check. http://www.bikemarket.com.my
If a customer purchase a product and pay through PayPal then the amount will receive by the admin. Now, if seller change the order status it will be changed. So, your seller have to get the confirmation from the admin about this then he should change the status. Now when the seller will withdraw the balance, he will be able to withdraw his completed purchase balance and admin will transfer his money manually from his PayPal account.
Please make sure that you are not missing something.