Although "Portrait d'une Femme" is filled with vivid, concrete imagery of the sea and sea-bound cargo, the actual merchandise traded seems to be conversation, which is much more intangible and harder to price. The poem's ending is especially strange in this regard, since the speaker seems to go through a lot of effort to transform the products of conversation into valuable objects that can be possessed, only to decide at the end that all this is nothing, and certainly nothing that the woman can own. Make up your mind, man!
Lines 29 and 30 are essentially a single iambic pentameter line that has been split into two parts. This represents, formally, a breakdown in the speaker's own ability to communicate and properly express his thoughts about the woman.
Both conversation and sea cargo are elaborate metaphors for sexual favors. This is why the speaker describes the woman receiving a "fee" (3) and providing "strange gain" (15) to her visitors.