Open your inventory. Do you have an overflow bag at the bottom? Do you have any holiday commodities there? You need to get them into your normal inventory before the vendor will follow through with the transaction. I believe the vendor adds your quantities in normal inventory + overflow bag to show you eligible items but will only be successful if all the commodities are in your normal inventory. Let us know if that helps.
That was it! I'm usually very careful to not hit overflow, I guess I just missed it. Still strange, the food item I purchased used the item I had in overflow, and even without that 1 item I had more then enough to buy the items.