As long as the phone is unlocked and it's a
GSM phone (unless you're sending it to Korea where they also use
CDMA technology) all you need to worry about is having the right bands for it to work over seas, you need a
GSM phone that supports three of the four major
GSM frequency bands, and will therefore work in most parts of the world.
The two major types of tri-band
GSM phones are European and Americas.
European tri-band phones support the 900, 1800, and 1900 frequency bands, They provide complete coverage in Europe, Africa, and Asia, and work on at least one network in most countries in the Americas.
Americas tri-band phones support the 800/850, 1800, and 1900 frequency bands. They provide complete coverage in the Americas, and will work on at least one network in most countries in Europe, Africa, and Asia.....
So when buying a
GSM phone make sure that it's the European version and make sure it covers the 900, 800, and 1900 band.
The unlocked phones you see on E-Bay are not attached to any provider, they're just letting you know that since it's unlocked, it will work with the major
GSM providers, Cingular, T-Mobile, Att, Rogers, etc...hope that helps.