I would exercise extreme caution buying invites from E-Bay. Reading through the OPO forums, seems like some people are getting scammed, but it works for others. Best bet is what
@J-Nice suggested and that's signing up to the OPO forum, be an active and constructive poster and somebody might give you one. It beats paying inflated prices for them(and the phone itself, which won't have a warranty) online. On the other hand, you'll most likely be waiting a while to get an invite from OPO themselves. I just got one yesterday and I signed up back on June 9th. If they give me any invites, I'll let people who are interested in this thread know.