Is it permissible to sell a device for decoding encrypted foreign channels that the questioner bought, to the owner of the shop who sold it to him, with the intention of getting rid of it, knowing that it will be sold for less than the purchase price?

Allah gives good tidings to those who repent, loves them, and transforms their evil deeds into good deeds. The one who repents from a sin is like one who has no sin. One should increase good deeds, associate with righteous people, attend gatherings of scholars, memorize the Quran and Hadith, and read the Prophetic biography.

Regarding the card: if it is used only for unlawful purposes, then the sale is invalid, and you may return it and take back the price. If it is used for both lawful and unlawful purposes, it is permissible to sell it to someone whom you predominantly believe will use it for lawful purposes, and it is not permissible to sell it to someone whom you predominantly believe will use it for unlawful purposes. If you do not know for what purpose the buyer will use it, then the criterion is based on the common use of it by people. If its common use is for unlawful purposes, then it must be destroyed.