When you get out of immigration in Poipet, there is a free shuttle bus that will take you to the bus station – DO NOT BUY TICKETS HERE. Particularly for a bus company called Rith Mony. They will rip you off and charge you US$15 for a $5-$7 ticket and put you on the slow bus that stops every 5 minutes to pick up passengers and freight on the side of the road. Do not be fooled by the high prices, the extra money goes straight into the pocket of the guy behind the ticket counter or his boss. A different price for foreigners and locals only exists when they are trying to rip you off. They get away with it at the Cambodia/Thai border in Poipet because it is all very confusing.
Across the road from the bus station there are plenty of other bus companies who are more ethical – buy tickets from there and just use the bus station for its clean bathrooms. Always ask for an express bus (these only stop in the major towns and once for a comfort stop) and DO NOT PAY MORE THAN US$7 to Phnom Penh or $5 to Siem Reap.
This happened to me yesterday when I was traveling home to Phnom Penh – they rushed me into a buying a ticket for a bus that had just left – I paid the $15 without thinking, and they took me to the bus on a moto, only to be told the bus had no space left. When I called their head office to ask about the correct price I was told it was a $5 ticket as it was the slow bus and that their office in Phnom Penh would refund when we got in. 9 hours later, the guys at the office refused to honour the refund, kept changing their minds about the price of the ticket (they gave me three different prices in 20 minutes!!) and were incredibly rude about it.
Extremely frustrating compared to how easy it can be in Thailand!