These sellers probably got their products for pennies. They might have bought in really huge quantities.
People will buy more if the item is cheap and shipping is free. Try to check the seller's feedbacks. Past sales may indicate the buyers tend to buy lots, not just 1 or 2. It could be a good selling strategy but risky, yes. Well, business is a risk in itself!
These sellers are mostly from China where everything's cheap including shipping. An example:Link hidden: Login to view