Joe,
You are not alone in these experiences. I've was contacted by an antiques dealers that was consigning a collection and wanted more info. After research I determined the 1890's broadside he had was a repo. I informed him, he thanked me profusely, and then listed on EBay the same day as authentic.
I've also made the mistake of making partial payments for items Fter executing a sales agreement and then have the seller completely disappear.
I have a lot of positive stories too, but these are valuable lessons I've learned over the years about the people you can cross paths with.
Chris
www.antiquefootball.com