Cambodia runs on US Dollars, thus the ATM machines dispense USD (although there are some where you can choose to have Khmer riels, but they are almost useless for tourists). US Dollar bills cannot have any marks, tears, or signs of wear. As for Khmer riels, they can be dirty and marked to the point you hardly recognize the value…
Since 2005, the ATM network has steadily increased. ACLEDA Bank has the most extensive network, with ATMs across the country and even in the smallest of places. ATMs of ANZ/Royal Bank are concentrated in the capital Phnom Penh and in tourist places like Siem Reap and Sihanoukville.
On the whole Cambodia is an inexpensive place, although it is a bit more expensive than neighbouring countries like Vietnam and Thailand. Of course you can reduce your costs by bargaining, although you shouldn’t overdo this as in Cambodia foreigners are quoted higher prices, but not nearly as much as in e.g. Vietnam.
Cambodia’s official currency is the riel, but you’ll see many people using the US Dollar to pay for everyday expenses. Don’t be fooled into changing dollars into riel at the land border crossings! The touts will tell you that the dollar is now not used anymore, so you have to change into riel at their ridicolous low exchange rate.
DO NOT BELIEVE any story like that.