http://revach.net/article.php?id=2152

Section: Halacha   Category: Tshuvos
Do You Do Bedikas Chametz In Places That You Sold?
Traditionally the Rov sells the chometz to the goy on the morning after Bedikas Chometz.  This means that during Bedikas Chometz all the chometz and the more importantly the places that you will sell tomorrow still belongs to you.  Do you need to make a bedika in these places?

The Mishna Brura (436:32) says it is a machlokes Acharonim.  The Mekor Chaim and Chayei Adam hold that you must do bedika in these places since they still belong to you.  The Binyan Tziyon says that you do not need to make a bedika.  He reasons that by arranging to sell it the next day it is just like the chometz you find during the bedika which you keep overnight and will burn in the morning.  The Chasam Sofer agrees with this leniency provided that you have left other rooms to do a proper bedika. The Piskei Tshuvos brings that if you sell on the 14th and are meikil not to do bedika in the places that you plan to sell, at the very least it is best to sign the document for the mechira and outlining all the places that you plan to sell the next day before you actually make bedikas chometz.

Although most people sell on Erev Pesach, the solution to avoid this problem is to sell before Bedikas Chometz.  This is referred to as “Mechiras Yud Gimmel”.  You must however be careful that if your Mechira takes effect on the 13th, since it is already sold, you may no longer go into these places or take things out even before Pesach begins. The Piskei Tshuvos says that some say, if you sell on the 13th you should write that the sale does not include the places where you will do a bedika, or else it is a Bracha L’vatala since all the chometz in the house is sold.  
 
Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.