For example, let's say there is a lot of people demanding furniture but very few sellers, how is the term to say "there's space" / good oportunities due to high demand + little demand satisfaction.
As Chad rightly points out, "seller's market" is often used to describe the general context of demand exceeding supply for an established "commodity" (i.e. - anything we're used to thinking of in terms of it being bought and sold, such as houses, second-hand cars, etc.).
You could describe this scenario as low market saturation. Market saturation describes the ratio of supply (especially number of supply sources) to demand: in highly saturated markets, most or all demand is met, leaving little room for growth. In an unsaturated market, there's a surplus of demand and relatively few competing supply sources.