If Rav Moshe maintained that supermarkets cannot sell chametz that is acquired during Pesach, why did he advocate the sale of supermarkets before Pesach?

For two reasons. First, the sale of a supermarket is halachically beneficial for the storeowners. By transferring ownership of large quantities of chametz that are on the supermarket shelves before Pesach, the owners are spared from violating multiple Torah restrictions. Second, the sale limits the status of chametz sheavar alav haPesach to the narrow window of chametz that is purchased by the store on Pesach, and thus, the turnaround time for the depletion of prohibited merchandise is shortened.