Julie, you're absolutely right. You can purchase a blackberry from ebay and pop your
sim card right in. As for issues you may experience, I've noticed that the contact info doesn't always transfer over. So you may have your old phone numbers, but not home addresses and email addresses.
Also, you don't know if the blackberry you're getting from ebay is under warranty. You will not be able to get insurance on it either. Only brand new phones purchased from direct or indirect T-Mobile dealers are eligible for insurance within the first 14 days of purchase.
Other than that, you should be able to configure a few things on a different BB and use it without a problem. No charge, just pop in the
SIM and adjust the BB to your liking.