Right before my Dad died in 1979, he sent his entire collection out for auction to Apfelbaum, Inc (http://www.apfelbauminc.com/).
He was a big collector. As far we know, he didn’t inventory it before hand, just put it on a pallet and shipped it. He frequently sent high value packages to various collectors and dealers and I (as a kid) asked how he could trust them. He said that if there was any kind of problem, word would get out and they would be unable to conduct any more business.
I see Apfelbaum is still in business.