Because if it's listed in the auction several times, how many more times can you tell someone "yes, the item is authentic".
Any yes, many of us original "from the very beginning" ebayers who are still around get insulted when repeatedly asked about authenticity on items. We take pride in our longevity and our feedback we've established over the years. Just as any other business would.
I don't mean to sound harsh, but that's just the way it is sometimes. And yes, I've blocked potential bidders who have asked me about authenticity of items. Why? Because, if they're unsure about an item's authenticity in the first place, who's to say they won't win the item, pay for it, and then decide they don't want the item after all or get it and claim it's not genuine. And then as a seller, I have the headache of dealing with that. Losing my money, and or the items, dealing with Paypal, etc. So for the assurance of myself and for the potential bidder, I've blocked them from bidding. It's like the old saying about starting off on the wrong foot with someone.
And I realize that people want to get a good deal on items, especially bags, but if you, as a buyer, have any doubts, then by all means don't buy it on ebay. And I just want to state again, that I don't condone, nor would ever sell a fake anything, and I've turned in more sellers that I can count, but please, as a long time seller, it is frustrating to be asked over and over again about the authenticity of an item especially if it's stated numerous times in the auction.