http://www.gsmworld.com/roaming/gsminfo/cou_th.shtml
But the rule of thumb is simple: if it is outside the Americas, the frequencies used are 900/1800, in some cases only 900 (usually in developing countries). GSM phones come in several varities:
1. Dual-band, either as 900/1800 or 850/1900. The former would work all over Europe, Asia, and Africa but not at all in the US. The latter the other way around.
2. Tri-bands. These are either 850/1800/1900 (works all over the US, and most of Western Europe and some other places) or 900/1800/1900 (work all over Europe/Asia and Africa, but may have more limited service in the US).
3. Quad-band. Work on 850/900/1800/1900. As long as you are near a tower, your set.
Yes, make sure the ebay phone is unlocked. YOu can also get phones that would work from dealers such as mobilebee.com.