Did you understate the value? I'm guessing you lied on the customs form and that's why they're holding it. (And if that's the case, count your blessings. There have been cases where items have been seized and sometimes destroyed because the seller undervalued it and customs assumed it was counterfeit.)
You can send them documentation showing what you sold it for. it would have to be the actual ebay transaction information and the paypal transaction info showing what you received as the sale price. You can't just state that you sold it for $x; you need to prove it.