The second chance offer:

Every once in while a non-selling item on the regular ebay site will be offered to the underbidders with a so called  "second chance offer", meaning that the seller may want to  consider your lower bid since the work did not sell at the reserve price.  Scammers have gotten the second chance offer official message from ebay and are directing it to unsuccessful bidders via their email address posted with their bids.
The first thing they want is your name and address and other personal details.
Then they direct you via a second message to use Western Union or some other non-recourse
cash transfer system to send the money, usually, to an overseas destination.

Ebay says to avoid this problem use only the official "my ebay" email mailbox at their site.  However, in this instance it is extremely hard to figure out what is coming from official ebay and what is coming from  a scammer's replicated ebay message.
Most legitimate ebay sellers will provide you with some means to contact them by phone or in some other direct way so you can verify the legitimacy of the offer and that may be your best method of verifying the "second chance offer"..