Is it permissible to sell face masks at their new, high price after the coronavirus pandemic, knowing that they were purchased at their old, low price, and that selling them at the old price would lead to a loss of capital and an inability to purchase the same quantity?

There is no objection to selling an item for more than its cost, even if it does not lead to a loss. There is no forbidden limit to profit, and the matter depends on the mutual consent between the seller and the buyer. If the profit is made without deception, then there is no objection to its amount.