Try this:
The bus station in Siem Reap is a chaotic place mainly catering to local traffic. It is located o¬n a dirt road off National Rd. No.6 about 4km east of the town centre. There is a Sokimex Gas Station at the road junction with Highway No.6. O¬nly buses to/from Phnom Penh and Battambang (via Sisophon) use the station.
Tickets can be purchased at the bus station (the ticket booths are a little way past the “station”), but if you buy them there, you will have to make your own way to the station. If you buy your ticket in town, the bus company provides a shuttle bus to take you to the station. The price for a tuk-tuk into town should be about $2-3. A motodup ride should cost $1. Be prepared to haggle.