Is a shoe seller sinful if he knows that some buyers will wear them for unlawful purposes, such as going to forbidden clubs and markets?

There is no harm in selling shoes or clothes to Muslims or disbelievers, nor to the obedient or the disobedient, as long as these items are not sold for disobedience or specifically designated for it. As for clothes whose sale is forbidden, they are those that violate the Sharia, such as clothes worn by immodest women, or those specific to disbelievers, or those with a cross on them. Other than that, selling clothes is permissible, even if a disbeliever or a disobedient person buys them. For Allah created for us all that is on Earth to help us in His obedience.