I've bought from nearly all of the dealers you mention above and have not had any complaints but that's because they know how to handle their business. You can't say that about every comic book seller, or every seller/retailer of any product.
As far as dealers who don't hang around the boards, I look at it as one of two ways, there are those dealers who don't do the whole message board thing and that's why you never see them on comic boards and then there are those who can't stand a little scrutiny, get pissed off and defensive when called on shenanigans and then leave, never to be seen again. They don't learn a damn thing because they have their heads up their asses, can't handle any form of criticism, and rarely accept responsibility. Nobody is perfect and mistakes happen. We all realize that but it's how they handle the mistake that matters. However, if their entire business model is based on not caring whether they screw people, over grade, or don't disclose restoration, then it's not reasonable to expect them to come onto these boards and mea culpa.