Check your eBay seller account for this offer, which has been posted before. Perhaps it's recurring?
It's not the most stellar one they've had, but if you're thinking of putting a high value item up in the next month or so this will be useful. Although it stipulates "No Insertion or Final Value Fees on your first sale" I've received (and redeemed) this same offer multiple times.
Expiry seems to be one month from the date you accept it.