You have the right to say no.
Exactly.
But look at your seller's feedback and his Detailed Seller Ratings. Second Chance Offers are usually legitimate, and if the seller has multiples of an item, such as an iPad or mass market jewelry, then no problem.
The negative and neutral comments are always a useful guide, but remember that while 2 negs in a 100 feedback seller is poor, 200 negs in a 100,000 feedback seller is darn good.