An extended metaphor is the basis a poetic conceit, which is a fanciful idea, an elaborate figure of speech expressed in extended metaphor or simile. In "Barter," the conceit, the extended metaphor is the personification of life.
Life is personified and made a merchant of loveliness: "Life has loveliness to sell." In the first stanza, life is pictured as selling "blue waves," "soaring fire," "children's faces." In the second, life sells music, scent, thought, love.
In the third stanza, loveliness is presented as the most valuable thing there is: one "shining moment" outweighs "years of strife." The conceit ends with the admonition to "give all" to buy the loveliness life has to sell.
There seems to be a naivete to the admonition that stems from the conceit that seems apparent in the notion that one shining moment of loveliness, say Rachmaninoff being played, can outweigh "years of strife." It may be that although the conceit and extended metaphor are lovely, they may be misleading.