Buying 8320 from eBay, Please Advise, on Rogers

Hi,

I always wanted a Blackberry and I'm thinking of spending 400$ on the 8320 from eBay. The wi-fi will help alleviate the disconnect while I wait for Canadian carriers to lower the cost of their data plans.

Anyhow, I have a few questions, if you can please answer some of them or complement each other's post, it would greatly help:

1) The phone from T-Mobile I'm buying is unlocked, it means I can just pop my Rogers SIM Card and use it?

2) Is the unlocking permanent? What if I wipe the phone, install apps or upgrade the OS?

3) To be able to unlock a phone myself, I would need to have a SIM Card from the original carrier right?

4) What is the manufacturer's warranty on purchased items over eBay?

5) Many sellers list OEM Phones, are they legal? Covered by any warranty?

Thanks!!!