Is it permissible to deal with a fish restaurant by selling them some types of fish, while leaving most types for the other seller they deal with?

There is no objection to displaying merchandise to those who deal with others so they may buy it. What is forbidden is selling over another's sale or bidding over another's bid after an agreement has been reached and the price has been settled, such as when a buyer says to the seller, "I will buy it," or when someone says to a buyer during the option period, "Annul it, and I will sell you something similar or better for the same price." However, before consent, it is not forbidden.