Certs? Broker? "Safekeeping?" Now that's funny.
OT: When you talk about "shorting" a stock, I think options contracts. Sell Short, etc... You collect a premium for agreeing to sell the stock at a price lower than current market value and at a later date. Then if the price of the stock rises, you loose your stock. If it stays the same, you keep the premium and the stock.
If you sign the contract, your stock gets shorted.