Several resources I saw introduce the notion of bid/ask spread when trying to price options in incomplete market, I don't understand why the notion is introduced since we are interested on the price that will be given by the seller of the option so why considering the bid-ask spread ? To be sure that the $$bid\leq ask$$ ? Did I miss something ?
• One generalization of equilibrium price is that there are two prices $P_B$ and $P_A$ such that if the market price was above $P_A$ everybody wants to sell it, if the price is below $P_B$ everyone wants to buy it, so these cases are in disequilibrium. But the prices $P_B<=P<=P_A$ are viable and can be considered no-arbitrage prices. In these theories the price is no longer unique. Jun 6 at 16:44