Spotting fake designers is hard whether it be ebay or at the local market. The best way round getting "stitched up" is to ask the seller to take detailed photos of the inside labels or certificates and then send them to you.
Usually if the items are fake the seller will either respond with a valid excuse such as "I don't have a decent camera". Or they could choose to ignore your message completely hoping that their silence will leave you still believing the item is real. If they do send pics to you, make sure the labels are stitched into the garment and not merely stitched on top of existing labels. Remember that 50% of the designer items on ebay are fake. The real items will have ample pics and state the item is 100% authentic. If in any doubt don't trust the seller and do not purchase the item.