You can buy a phone from ebay for use on Bell Mobility however it MUST be a
CDMA Bell phone.
GSM does not work on the Bell network.
You can take it to a store to have your number programmed into it - they charge $15 for this service. Then call Client Care and have them change the
ESN on your account. This is free and takes effect within a few hours or sooner.
Depending on who you get when you call into Client Care at Bell they may (and they can) instruct you how to program it over the phone and that doesn't cost you anything.
I would HIGHLY recommend that you obtain the
ESN from the seller prior to purchasing a phone from ebay. If the phone was reported lost or stolen then you cannot use it on the Bell network.
Search google for ebay + bell mobility and you will see a page about buying Bell phones. Can't post a link sorry I'm not allowed yet. Haven't made enough posts.
Good luck.