http://revach.net/article.php?id=2166

Section: Halacha   Category: Tshuvos
Buying From A Store That Sold Its Chometz On Pesach, But Also Sold Chometz On Pesach
There are many issues as to how the Goy can halachicly “buy” the owner's Chometz on Pesach.  The Mishna Brura (448:17) says that even if the “Kinyan” made by the goy is a less than perfect Kinyan, nevertheless this chometz is permissible after Pesach since, among other reasons, the Jewish owner by selling it to the goy has shown that he doesn’t want it and removes it from his domain.

If this is the mindset we are relying on then what is the Halacha with a non-religious storekeeper who is not interested in really ridding himself of the chometz and is doing so merely to keep his religious customers coming after Pesach? The Piskei Tshuvos (448:20) says that in this case the Sdei Chemed paskens based on the Maharam Shick that if the storekeeper sold any of the sold chometz on Pesach the whole sale is null and void from the beginning because he didn’t really have in mind to part with his chometz.  However, in the case where the storekeeper despite the fact that he thinks the sale is a joke, abides by the terms of the sale and does not deal with the sold chometz on Pesach then it is Mutar after Pesach.

He brings others that say that even if the storekeeper did not abide by the sale and took from the sold chometz on Pesach the remaining chometz is still Mutar after Pesach.  The reason is that today the sale to the Goy is made in a perfectly valid way and the goy actually agrees to pay full price for the chometz, therefore it doesn’t make a difference what the storekeeper thinks since it technically belongs to the goy.  According to these shitos, if the storekeeper sold chometz on Pesach then he is a thief who stole from the goy but after Pesach the rest of the chometz is Mutar.  
 
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.