You would need to contact Bomb Factory:
www.bombfactory.com
But if you didn't go through any kind of transfer of ownership, don't have the receipt from Bomb Factory or one of their authorized distributers, the installers and everything else that comes with purchase, and can't arrange for any of that with the seller, then Bomb Factory is probably not going to able to do anything for you.
So you more than likely got ripped off, or more to the point, the seller misrepresented what he was selling you. It doesn't mean you did anything illegal, just that you and the seller have not gone through the proper steps to legally transfer ownership of software that may or may not be their''s to sell in the first place. It may be a legal version, and depending on Bomb Factory's policy, they may allow a transfer of ownership. But they may not if the plug-ins used a challenge response rather than a key disk or iLok. If that were the case, then the original seller might still have the challenge response authorization on their own hard drive, which I would imagine would make BF reluctant to issue a new one to you.
It's kind of the same as if someone owns Photoshop, it is registered to them, and they have all of the manuals and installation CDs. Just because they install it on a computer, reuse their serial number and sell that computer to someone else doesn't mean that it is a legal copy or the legal property of whoever buys the computer.