$50 for a Storm? Has to be a problem. Don't know anything about Centennial, Are they CDMA or GSM? Personally, for that price, I wouldn't touch it.
If they are a GSM carrier, you will need to get an unlock code to unlock the GSM side. Not sure what that will cost though and you will only be on the EDGE network.
If they are a CDMA carrier, then you need to check and see if they will activate a Verizon device on their network.
ANd you need to read the FAQ article on buying a used BB from eBay. Even though you aren't buying it there, you can run into the same problems if you do not know what you are doing with a BB.
Also make sure you read the posts in the 9500 section. Lots of insight on both the Storm and Storm 2. Buying a BlackBerry off eBay - BlackBerryFAQ