My thought is that if someone has purchased a bag, and feels it may not be authentic after the sale, they are right to come here, using the proper format of course, and ask for authentication. At least it may give them some recourse with the seller and/or their credit card company.
If the bag was authenticated here before the sale, unless there are errors the buyer can point out that are different from the listing, I would expect the second request to be ignored.
This would be a good way to proceed IMO.