It's pretty rare to use cash these days, but sometimes you need more than the ATM will allow you to withdraw. If you're buying used equipment with Craigslist, you can easily need quick access to more than $300 or $500 in cash. At most banks, this is a problem -- they don't want to open your account up to whoever happens to have your ATM card and know your PIN; if somebody is using a stolen card or a fake card, the bank will never recover the money.
A good workaround is to go to a bank and use your card for a cash advance. You don't even have to go to your bank -- any bank can accommodate you, although they may charge a fee for this service. Lifehacker notes that several banks offer cash advances for free or for a minimal fee. Readers also note that you may have a few other options, including getting cash back at a grocery store, and asking your bank to raise your ATM withdrawal limit.
Of course, all of this depends on what your bank allows. In some cases, you won't be able to get more than $500 or so of cash per day no matter what. As always, it's best to find out how your account works long before you need cash.