Buying phones on ebay?

All right, I have a lame phone that came free with my plan with Verizon. I really want a new one, but my contract doesn’t end until fall 09 and buying directly from Verizon is ridiculously expensive. A friend said I could get a phone on ebay, take it to the Verizon store and then have them put it on my plan and whatever.
I’m just wondering if anyone has actually done this, because I don’t wanna spend a bunch of money and then have Verizon tell me I’m SOL.