Will I be sinful if I sell a satellite receiver (receiver) to my friends, knowing that I fear it will be used to watch unlawful things, and knowing that I cannot cancel the deal?

There is no harm in purchasing the two devices that you were commissioned to buy for permissible use, and it is not necessary for the contract to be in a name other than yours. If the buyer misuses them in disobedience, then the sin is solely upon him, and you bear no burden, because you did not intentionally or directly assist him in the disobedience. You have done well in offering advice and admonition.