There was a post a while back on releasing PINs, but I can't remember where exactly, and I can't say if it works or not. If I find it, I'll post it, otherwise just do a search for releasing PINs.
But note that this is one of the dangers in buying a phone off of eBay. If the seller said that the phone was unlocked, then they should have ensured that it was truely unlocked. Since they didn't ensure that this was the case, you could say that the phone was sold under false pretenses and you could ship it back and get your money back.
Anyhow, hopefully you're able to get a hold of the original seller, otherwise you'll have to see if you can find that post or return the phone.
Sorry I don't have better news.
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