Can someone explain to me?

Do I need to trade with someone in the same server/realm?

Say my toon is in server A, a seller is play in B, how do we trade a token/game time?

Tokens are region specific.

You buy them with gold, gold amount will be the same for every server on the same region.

Just go to the AH and buy it.

If you want to buy one vs fg, whoever sells it to you will check gold price and trade you the gold amount that equates to 1 (or more) token for you to buy a token with.

gl.

The person selling the token (gold on a specific server) will tell you server and faction to create a level 1 (orc or human usually) and you will meet at the AH. They will trade you the gold. You buy the token from the AH. Redeem and you have 30 days of game time.