Wirelessly posted (8800: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0) BlackBerry8310/4.2.2 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102)
Read the FAQ about buying off ebay, bottom of this main page. You need to have the previos owner release the PIN from their account. You can ask the carrier to do so yourselfn but they don't have to, so ask nicely!
http://www.blackberryfaq.com/index.php/Main_Page