The title promises something shady, but the poem itself is oddly gentle. The speaker is awake late at night, turning up small discoveries so often that they can't keep track. The "arms dealer" seems to be the speaker, trading in these finds the way a dealer trades in goods, though what's on offer is strange: a ladder to the heart, a good red tune, a plot.
Each find is a kind of tool or story. A ladder offers a way in to feeling. A tune brings warmth and pleasure. The last one turns darker, a plot to make war look "greater than money," as if the speaker has stumbled on a scheme about what drives conflict. That ending sets the title's irony in sharper focus: the same night-time mind that collects tenderness also finds the machinery of violence.
The mood is quiet, restless and a little uneasy. It feels like the half-lit state of sleeplessness, when ideas arrive at random and the line between wonder and worry blurs.