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I suppose it is possible that the sea shells may have scratched the paint in the bed of the dump bed, and they want to re-paint it. You should ask to see the repair estimate to verify what the exact damages they are claiming.
Any time a new vehicle is driven off the lot, it is no longer new. If, as you suggest, the sale was cancelled due to a mistake on their part, they should bear responsibility for this. Additionally, the type of trivial cosmetic damages I suspect they are claiming should not significantly affect the value of the truck for commerical purposes.