I think some replies have touched on this.
Strictly speaking if there are no buyer's and no sellers, there is no market. Strictly speaking if the price is low enough you will find a buyer and if the price is high enough you will find a seller. What will actually happen when liquidity drives up (meaning there's no buyers/sellers at the current price), the spread will widen until such a time that the offer is low enough to entice a buyer or the bid is high enough to entice a seller. This is effectively why lower liquidity instruments (where there are a smaller number of people interested in buying/selling at the "price") have a larger spread. As there are more buyer/sellers, there are also more people willing to buy or sell at different price points. More price points will get you to tighter spreads.
In short, there's never a lack of buyers or sellers, there's only a lack of buyers or sellers at a particular price, so you search the "price" both up and down until you find a buyer and seller and that defines the spread or the BBO.
