When placed in a home, this board can be used to put items up for sale on the Broker, but you can not buy things while inside your home.
In addition, it is used to place special furniture in your home in order to sell to those who visit it; doing so lists your address on the broker and buyers can avoid the broker fees. For more information see the Sales Display page.
There is at least one merchant in every city that sells Market Boards if you need a new one or buy more than one house. The NPCs are typically close to the starter housing in Freeport, Qeynos, Neriak, Kelethin, Gorowyn, and New Halas.
For example, you can buy it from Vhishall, the furniture merchant, in the City of Freeport, or from Minda Broadfeet in Qeynos Province District.