The title of the Sara Teasdale poem hints to its true meaning. The word "barter" means to trade one thing for another. Teasdale's poem illustrates the belief that the good things in life do not come for free; they must be purchased or bartered by giving up other aspects of a person's nature. The first two stanzas contain a positive statement of things worth living for: blue waves, children's faces, the scent of pine trees, arms that hold. But the final stanza offers a warning about the consequences of giving up too much "for a breath of ecstasy."
I disagree. I don't think the poem offers a warning about the consequences of giving up too much. Rather, I think the poem suggests that there is no price too high "for a breath of ecstasy." The final line tells us, without qualification, to "Give all you have been, or could be" for the moments of beauty that life has to offer.
The first two stanzas list a great many lovely things that life has to offer, and the first line of both is that "Life has loveliness to sell." The implication, obviously, is that there is a price for these things, but in the final stanza, the speaker says that we should "Spend all [we] have for loveliness, / Buy it and never count the cost." In other words, these moments are worth giving up whatever else we have, and we shouldn't hold back anything when we have the chance to purchase the loveliness that life has to "sell" us. So, yes, we will have to barter, but the speaker says that what we get is of far greater value than anything we must trade for it.