Is it illegal to buy products in the internet and resell the products at a store?

Too broad a question to answer yes or no. As a general rule you can buy and re-sell almost anything. However you cannot hold yourself out to be a factory-authorized seller or franchisee of a brand-protected product unless you actually are.

For example: you cannot present yourself implying that you are dealer in Louis Vuitton purses, if you are not.