How does the buying at wholesale prices and selling at retail prices help you make profit?

An example would be if you bought a sofa at a wholesale distributor for $300.00, then sold it at a mark-up of 2.100 percent, you would subtract out the cost of transportation,i.e. $25.00, and your profit is $305.00. You would have to subtract any overhead you might have also, if you own or rent a storefront.