Clearly this person feels like the item is worth less than you do. Maybe this is a person who wants to move items quickly through their retainers so they can free up those market spaces to sell other items, or maybe this person just thinks the market is inflated and that the item could be made cheaply and has no business being as expensive as it is.

Don't just cry that this person is undercutting your selling price, diversify. Find a new niche to fill, or find a more efficient way to make a profit on this same item. This is basic capitalism, and that is the economic model that rules our current market board system.