I very rarely have participated in selling anything short - however, it is a very much needed component to form a proper market. You can't just have investors lining up on one side of the company's balance sheet. It would be too easy for that company to manipulate their worth and nobody would be the wiser.
How are they piling up on one side?
If the stock is going for 40 and you want to pay 30 you put in an order to buy for 30.
Pretty simple.