Example: I find a patio furniture set at Big Lots for $400, I list it on a website for $600. When it sells, I go buy it for $400 and deliver it to the buyer who paid $600.
It's that simple.
In that example the profit would be the difference between what I sold it for and what I purchased it for. You can repeat this over and over again when the price discrepancies allow for a profit. Online we can do this and never even touch the product.
You list a product on your store, when it gets sold you buy it from your supplier!