Not eating chametz that was sold to a non-Jew

This has become a very popular chumra in the Charedi world in Israel. However, it is all a fake. It is practically impossible to keep this chumra as manufacturers can put whatever they want on the label and no one checks whether it is actually true.

Kashrus organizations do not see a difference between cake sold halachically to a גוי and cake made after Pesach. Therefore there are no grounds for relying on the manufactures declarations. In fact, the kashrus organizations stance is usually to permit the manufacturers to write on the package whatever they want.

In other words the Kashrus organizations that almost all Charedim rely on like the Eidah Hacharedis, R' Landau, R' Rubin and others hold that this is a chumra with no basis and therefore have no issue with the manufacturers writing whatever they want. So even if the package says baked after Pesach you have no way of knowing that this is true.

Some people rely on checking product codes which tells them when the product was made. However, this is not that useful. All it says is that the product was made after Pesach. However, every Chometz product has chometz ingredients in it (at least flour which is most probably chometz because it was washed) and the consumer has no way of knowing when the chometz ingredients were made. For example even if you only buy cookies that were made after Pesach you have no idea what flour was used. It is very possible/probable that the flour used to make the cookies was chometz and was sold for Pesach. Likewise for all of the other Chometz ingredients.