I've a Blackberry 8520 on contract for two year expiring this September, I've used the contract sim card on my iPhone and I want to sell the Blackberry 8520, how do I erase all data, BBM messages and contacts?

Also since the Blackberry 8520 is on contract, can the buyer check my information such as name, address and others?

Do a Security Wipe on the device. Once you have wiped the device, no one will be able to access your information.

How to do a backup and security wipe of Blackberry... - BlackBerry Support Community Forums

For wiping your handheld follow the below steps -

1. Go to Options
2. Security Options
3. General Settings
4. Click Menu button
5. select Wipe Handheld
6. type in blackberry to proceed