Second chance offers can be totally legit. It happens when the original winner of the auction backs out for one reason or another. Then the seller can offer the item to the next highest bidder, hoping to sell the item for a few dollars less without having to relist it. Relisting costs extra money, while a second chance offer does not. I've given potential buyers second chance offers on my auctions, and I've been given (and have taken) legit second offers as well.
The key is that the second chance offer has to come through the mail system on eBay. On your "My eBay" page you'll see a link on the left that says "My Messages". The offer has to appear there for it to be legit.
If you get an email that does not appear on your "My eBay" messages page, it is not legit. No matter how real the email may look, avoid it unless it comes through eBay's messaging system.
However, as I said, if the message *does* appear on your "My eBay" page and has come through eBay's system, then it is real. If they want to settle the second chance outside of eBay, or have you email another address outside of eBay, avoid those.
But this idea that *any* second chance offer is not legit is incorrect, there certainly are legit second chance offers.
That said, I personally would avoid any auction by a first time seller, and a second chance offer from a first time seller. It has less to do with second chance offers than wanting to buy from someone with a proven track record.