Need Help with Legit check

Hi everyone, I’m in a frustrating situation and need some help from those knowledgeable about authenticating rare designer pieces. I’m a fashion stylist, and I recently offered one of my pre-used Balenciaga items for sale — a limited-edition denim jacket (1/10 edition). Unfortunately, the buyer’s team is questioning its authenticity because they couldn’t find the item online or in any official database, They also mentioned seeing counterfeit versions of this design on Grailed, which added to their skepticism.and I don’t have proof of payment since this item came from a celebrity client I worked with.