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.
You would say that it's a "seller's market", meaning that the most of the advantage is with the seller, who would be more free to set a higher price.
The opposite is, logically, a "buyer's market", when there are lots of sellers and few buyers.
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.