Yes, true. If the restorer is unknown, then so be it.
BUT -----
In many situations (again, not all), a seller, dealer or auctioneer does know this info (or better yet, a Dealer/Auctioneer has sent a poster off to be restored him/herself) to be worked on, prior to auctioning it.
Why then, is that person described, subsequently & generically, only as "the restorer?"
Don't they deserve credit for the work they have done?
I think they do.