Minecraft wiki entry for Superflat_survival, associated text: Wheat Seeds: Don't buy it from wandering traders, but use bone meal on grass blocks and break the grass, which yields at least one wheat seed.
Carrots and Potatoes: A wandering trader sells a carrot and a potato for one emerald, but you can also get these items by killing zombies.
Vines: Wandering traders sell vines for one emerald; however, vines also grow on large jungle trees.
Flower: Since the biome in classic flat preset is plains, by using bonemeal you can get poppies, dandelions, azure bluets, oxeye daisies, cornflowers, and some areas can grow tulips. So buying flowers only useful if the flower is blue orchid, allium and lily of the valley. Also buying flowers for flower farms just to get dye is not recommended.