Optimists like to call a market decline a "buying opportunity," and, with stocks down more than 50 points over the past two days, Citibank is making it easier to purchase stocks. The bank announced Wednesday that customers will be able to buy stocks, bonds and mutual funds through its 1,800 automatic teller machines in five major cities. The service is available today in New York City, Chicago and Miami. It will be in San Francisco and Washington by the end of the month. Citibank customers can use the ATMs to find out a stock's latest price, and then issue a buy or sell order. The money is automatically deducted from, or added to, the account. Initially, however, you will have to have a fat $100,000 balance with the bank to use the service. But at least your stocks, bonds and other investments can be part of that total, if you've got them.