I have had bidders retract their bids within the last 5 minutes.
Technically you are not allowed/supposed to do this, but the ebay system does not prevent bids from being retracted within the last 12 hours.
I think basically it's a rule that is not enforced.
If you end up winning, and the bag is not authentic, you can choose not to pay but you may risk getting a negative. You should try to mutually agree not to complete the transaction and let the seller either get their listing fees back or have them sell to the next bidder.
I state on my auctions that I will accept withdrawal for buyers remorse before the auction ends. But once I send the bag, I won't accept a return for buyers remorse. Just easier for me that way. I'd rather eat the listing fee than have to deal with refunds.