However, French customs seize items regularly and take the matter seriously.
This.
They will investigate. They will seize and destroy the item. They may very likely press charges. And since /buying/ a counterfeit item is a crime as well as selling one, and since importation is against the law and very, very heavily inspected, the buyer could face charges as well if they try to import it back into the country, return or no.
Not a situation I'd like to face, since I very much enjoy traveling to France. And since you cannot be required to perform illegally, Ebay cannot make you return an item to a seller which could result in you being charged with a crime. Destoy it? Sure. Return it to Ebay for disposition? No prob. Turn it over to LV with a receipt to show Ebay that it was remanded to the holder of the copyright? Absolutely. Send it back to the seller? No way in he!!.
You should be out no money on this transaction, except the cost to send it to Ebay (domestically, so it should be cheaper than internationally!) if they require you to send it to them. And possibly an authentication, if they require that.