If I cannot upgrade my phone yet, but want to get the Curve when it comes out for Verizon, I can buy it from EBay, right? Then what do I do? Just bring it to the Verizon store and they can enable everything and give me the Blackberry service? Has anyone done this?
Just call up Verizon and give them the IMEI/serial number of the phone and they will activate it.
Slightly OT: As dumb as it might sound, this is one of the reasons why I left Verizon. I go biking, skiing, hunting a lot, and hated to bring my nice phone, and it would've been WAAAAAY too much hassle calling up Verizon to change the service from one phone to another and back to the other. With T-Mo (and AT&T. Don't know about Nextel (Sprint) anymore), just pop out the SIM and pop in the new phone. Done.
If you are buying secondhand, and you are going to use verizon, buy a verizon branded blackberry. Also see the article in blackberryfaq.com on buying a used blackberry through ebay. Good advice on buying a used blackberry.
I second the advice on reading the "Buying a used BB from eBay" on the FAQ. You can really run into a lot of problems if you don't know what you are doing. It isn't as simple as buying an ordinary cell phone.
Also make sure you buy a Verizon branded device, otherwise it won't get activated on their network.