Unlocking does very little for a CDMA device. Currently, Sprint will not allow a non-Sprint phone on their network, even though it is essentially the same model as they also carry (i.e. 8830). Verizon has the same policy, although both are saying they might allow non-company branded devices on at some point in time. Every once in a while we hear of it happening, but not often.
In addition, since you bought a used device, please read the FAQ article about buying a used BB from eBay. Even if it didn't come from eBay, the message is the same.You can run into problems if you don't know what you are doing. Buying a BlackBerry off eBay - BlackBerryFAQ