eBay's claim is that offers to buy/sell outside of the eBay environment leave you without their
protection - the thing is I don't know what that really means.
Getting back to your question though; the high volume eBay seller account that I help manage has become aware of a lot of bogus Second Chance Offers to failed bidders recently and I'm sure that
bad offers are being made. In our case, we have noticed that the bogus sellers have attempted to impersonate us, a seller you might readily trust, by using similar eBay account names/email addresses amongst other things.
eBay's fees are pretty good and you have to ask yourself the simple question "why the hell isn't the seller just offering their wares at public auction" if the deal is real. It's beyond me to say that it isn't genuine and if you decide to persue the sale then I'd urge you to use some form of escrow.
Oh, and lastly, it is against the eBay rules and you should consider reporting it, if not for your benefit then for the benefit of others less suspicious than you.