Hi! The first auction, item 280291527530, is authentic. (The clear plastic bag and the tissue is how the bags arrive at the boutique before they are put on display. Fakes usually have plastic-wrapped handles.)
When using eBay, always, always, ALWAYS ask for pictures of the actual item. The last 3 auctions use stock photos (in other words, taken from the Longchamp website), and there is no way to tell whether the item you will receive is authentic or not.
In general, watch out for sellers in Turkey and China and Hong Kong. There are many fakes from those countries on eBay. The best way to check for a fake is look at the leather up close. Real longchamp Pliage bags have a crisscross diamond grain on the leather. The leather should not look perfectly smooth, scaly, or spotted.