The rhymed iambic tetrameter, the regular repeated sounds and stress patterns, helps make "Barter" sound song-like. Teasdale is kind of a rhythm junky, she likes her iambs. Let's take a closer look at how she makes those toe-tappin' lines, and just what the heck we're talking about here.
So, an iamb is an unstressed syllable followed by a stressed syllable. (It sounds like da DUM.) And iambic tetrameter is when you have that pattern four times (tetra- meaning four) in a single line. For example:
all beautiful and splendid things (2)
Hear it? Try reading it aloud. You should hear this rhythm: da DUM da DUM da DUM da DUM.
Teasdale follows this pattern pretty strictly throughout the poem, but there are some exceptions. Take a look at line 1:
Life has loveliness to sell
What's wrong with this picture? Yup, that first unstressed syllable is missing. This is called a headless line (or for those who like more technical sounding terms, an acephalous line—Google it). By cutting off that initial unstressed syllable, the word "life" gets more stress and attention, which kind of goes along with what Sara is talking about—paying more attention to all that loveliness life has to offer. Teasdale uses these headless lines when she wants to give some added oomph to the first word. Lines 3, 4, and 9 are good examples. She really makes us pay attention to those first words—"blue," "soar," and "scent"—all very sensory words.
The other element that gives this poem such a song-like feel is the rhyme scheme. Take a look at the last word of each line. "Barter" follows a strict end rhyme pattern of ABCBDD. That final DD really makes the rhyme stand out and makes each stanza feel like a completed unit. When we hit that DD, we know we are in for a transition. It's a signal that tells you the end of the stanza is coming—kind of like a yellow traffic light.
You probably noticed that the second and fourth lines of each stanza are indented and wondered to yourself, "What gives?" Well, this structural element makes the first two rhymed lines in each stanza visually stand out, strengthening the connection between those lines and making us even more aware of the already strong end rhyme. It also mirrors the kind of back and forth sway of a dance—remember, Teasdale's all about the music.