Where do they even buy things so they can sell?
People/Companies that buy in bulk usually sell to distributors and don't have an actual store front(because that is the people that they are selling to).
They buy their products from the companies that make the items. They have the warehouse capacity to make the minimum order amounts that the bulk buyers/sellers need. They buy in huge bulk amounts so that they can get the best deal available to sell on so others can do the same.
They do deals with distributors.