I have a similar story. I bought a great looking Santa Fe Diesel Locomotive in C8+ condition on Stout Auctions. The description never said anything about being a dummy unit, it said 'diesel locomotive. The picture underneath shows rollers, but there was no box to see the side flap description. I got it for a great price, at which point I looked up the model number on Greenbergs. There it clearly said it was a dummy unit. I contacted Stouts before they issued an invoice and told them I wouldn't have bought it if I knew it was a dummy unit. They looked into it and said that yep, their description was lacking, so they cancelled the sale. Stouts have been helpful on several occasions.
I saw the unit in their next auction where it was clearly worded as a dummy unit.