Is it permissible for an employed salesperson to purchase goods from the store where he works for himself, paying for them after selling them at a higher price?

If the shop owner permits you to buy from yourself on credit, then it is permissible for you to buy the goods and sell them at a higher price. However, if he does not permit this, then it is not permissible for you to buy and then repay the price after selling, because it is not valid for an agent to sell at a deferred price without permission.