Unless the seller represented them as something different than “what you see is what you get”, I think you have learned an expensive lesson. If you know the guy he may take pity on you and return some, but it is likely he wouldn’t have known about it anymore than you did (or even the tire place when you dropped them off). Just my thoughts….
I don’t have much experience with reusing older tires but what little I have had, has not been good. Tires don’t hold up well to being removed then the rims if they have been there for years are usually worse shape than you expect/hope.
On a much (much, much) smaller scale, I have a CC Original with two great looking agg tires on them, just older. I had the idea I would remove the tires, clean and paint the rims, use some Miller tire paint on the tires, reassemble and would be good as new… wrong, tires were brittle, would have likely lasted much longer had I not messed with them.
Sorry about your surprise and best of luck,