Please check out the FAQ on buying a used BB. It isn't at all like buying a used cell phone. There are a number of issues you might run into where only the seller can fix, such as not closing the BIS account, account not in good standing, device reported lost or stolen, etc. Plus make sure you get the right frequency bands and right technology (GSM or CDMA) for your carrier. You could end up with an expensive paperweight.
You also need to make sure that the carrier you wish to use has BB support and offers a BB dataplan. Without a BB dataplan, all you have is a phone, SMS and a PIM. No Internet, no MMS and no email. In addition, if they do not have the 8310, you will have no tech support to help resolve any issues of using a UK or US BB on their network. You should also ensure the carrier allows other phones on the network as well.