Not sure that you can get a BB for free, at least not the new generation 8700s. $299 is the upgrade price I think. I got my first one for that doing an upgrade from a Treo 650. When I had to replace the 8700 due to water damage, I had to pay the full $450.
Buying on eBay to me seems very risky. First you need the right phone on the right system (CDMA vs GSM) and you also need the PIN released so you can use it. Plus you need to make sure it actually works as advertised!