Well if its locked, it can only be activated on the carrier it was meant for originally.
If not locked an is compatible, it is up to the carrier you are hooking up to if they allow foreign
esn's or not on thier network some are picky like Telus, they do not allow other handsets on their network.
Call your carrier an ask if they allow unlocked phones to be used or not on thier phones network, an if they do, get the
ESN off of the seller an ask if it is allowed, it may be stolen, still in someones names etc... Just to make sure it clears.
Hope this helps.
Peace
WR