I guess people feel safer buying preloved from Mulberry directly, particularly if they're not aware of authentication services or aren't confident enough to authenticate bags themselves. However given the confusion re bag names/leathers etc I wouldn't be particularly confident buying direct.
I'll never forget, years ago on here, the sorry tale of a Ledbury and a Smithfield being sent to Mulberry for authentication. The Smithfield was declared genuine, the Ledbury fake. Little Ledbury was destroyed by Mulberry and Smithfield returned to her owner. But it transpired that Ledbury was in fact the genuine bag, and Smithfield the fake! I know that Mulberry hasn't offered an authentication service for years but still, it doesn't inspire confidence does it.
Oh for the days of Mulberrymad and LHB! At least we have NPN still. I'm a dab hand at authenticating the older Darwin/NVT bags myself, never bought a dud yet, so there's that too. I think I'd trust my judgment on those bags over Mulberry's, given they don't even seem to know the names of the bags let alone anything else!