Blackberry use a complete different email system to most other phones , in as much as its permanently live, like a push system.
So you get emails straight away.
Make sure that any blackberry you buy off ebay has been de-registered from blackberry, otherwise you wont be able to use the email system and alot of the other features that will be tied to the handset account.
You will then need to register the handset to you.
This has nothing to do with your Service provider, this is with blackberry themselves.
If its still registered or stolen, you cant use half the stuff on it!
Blackberry GPS etc.
If flashed you will be able to make calls & receive, be able to text, but thats it (no email).
WE bought an 8311 curve off a car boot sale, cheap and had to get it flashed for my son to use it.
Its not a bad phone and durable as hell, but he cant use email or the GPS fuctions as its never been de-registerd!
I had it unlocked at the same time and the handset has not been barred, so i'm assuming it wasn't nicked, just ex contract on O2.
So i would ask the question about registration before you commit.
If the seller does not know what your talking about, then he is not the owner!