I recieved an email and a letter from Gibney Anthony & Flaherty LLP?

I had purchased two jerseys from the same seller over ebay. He owns a store that specializes in NHL jerseys. I got them in the mail and the sizes where not right. So I decided to resell them on ebay. Next thing I know, I recieved an email and a letter from Gibney Anthony & Flaherty LLP saying that I am illegally selling merchandise (the two jerseys). That the two jerseys are fakes. I sooo did not know that. How could they tell anyway from a picture on ebay. I can't tell a fake from a real one anyways. Anyway, the letter states that they are demanding that I turn the jerseys over or I will be held liable for all sorts of damages. They are requiring that I give them the sellers information too. My question is, how legitimate is this? I am scared. Do I have too turn the jerseys over? I don't know what to do. I am a single mom trying to survive. Help!!!! Do I have to comply? What will happen if I do not respond. Is this just a scare tactic.