Whichever route you’ve used to arrive at the border, everybody has to disembark and walk through customs – there are no direct busses and no taxi driver will be willing to drive across. Queue at the Thai immigration to get stamped out, if you have a visa already you can walk through to Cambodian customs and get stamped in. If not, there is an office before the casino where it’s quite straightforward to obtain a visa, although plenty of touts will offer to fill in the forms in the hope of a tip. You could ignore them if you like. There is no overcharging for visas at this crossing anymore, but remember they only accept US dollars, and may refuse those that are torn. Now walk past the casinos (set up in the no mans land between the two countries, and capitalising on the illegality of gambling in Thailand) and more queuing before finally making it into Cambodia
Immediately after the checkpoint is a roundabout, and there is a free bus from a stop here which will take you to the international tourist bus terminal. This bus station is very far from town, about 20km, and busses and taxis serving it are expensive – the bus is nine dollars, now consider that going in the opposite direction from Siem reap it costs 3.50. Taxis cost a ridiculous 80 dollars, when a fair price is 30. Once you are at the bus station these are the only prices you can leave for, haggling is impossible. So I would say the best thing is to buy an all in one ticket from a Bangkok, because you wont have to pay for the bus here. Despite its former reputation as a scam bus, these days it seems to be the cheapest and fastest route to Siem reap. The bus drives directly to Siem Reap and only makes one stop in a three-hour journey.
If your plan is to travel by taxi, DON’T get on the free shuttle bus, but attempt to pick up a taxi in poi pet itself, away from the transport cartel. This can be difficult as the police are involved in the operation of these ‘official’ taxis, and are ordered to stop taxi drivers picking up from the border. But if you walk past the immediate area outside immigration, around the roundabout and up the road, you can find a taxi willing to take you.
Siem Reap to Poi Pet
All the official rip off rates only apply to coming from poi pet, travel agents sell tickets to Poi pet for $3.50. They might say there is a five dollar VIP bus, but as far as I know there isn’t, they’re just trying for more money. You can book a taxi for a fair price of 30 dollars. Travel agents also sell a combined ticket to Bangkok for as little as 10 dollar, this will be in a cramped minivan in Thailand, but if you want to go to Ko San road they do at least take you directly there. Otherwise there are lots of transport options as listed here. There are actually two places that busses leave, from Aranya Prathet town’s bus station which has many more busses going to more destinations, also there is a small bus station at the market. This offers less frequent busses that will not be express services. It’s best to go to the main bus station.
Or you could skip all this drama and fly