In "Barter," Teasdale uses primarily natural images as examples of loveliness. She shows us that loveliness is all around. With all those natural images as examples, we get the sense that living life in appreciation of loveliness and beauty is the right, the natural, way to live. Shmoop suddenly wants to go camping—anyone up for s'mores?
Teasdale uses nature imagery because there is an underlying sense of danger in nature and this makes the poem more complex and interesting. For example, Teasdale's image of the waves against the cliffs is beautiful, but you wouldn't want to be in that ocean. You surfers in the crowd know that big waves and rocks can be a nasty combo.
Teasdale uses nature imagery because it is extremely accessible for the average reader. All of her nature images evoke sensory, positive responses in the reader, helping her convey her message that life is lovely.