What is the ruling on an I'tikaf retreatant calling a restaurant to order food, or using food delivery applications that list meal prices, or using a laptop for stock trading? Does this fall under the prohibition of buying and selling in the mosque?

A person performing i'tikaf contacting a restaurant to order food does not affect the validity of his i'tikaf, because it is less impactful than going out to eat and drink. However, it is better for someone else to arrange his food. If he wishes to make a call, it should be outside the mosque. As for using a mobile phone for trading stocks or other forms of buying and selling, it is not permissible inside the mosque for someone in i'tikaf or anyone else; this is due to the Prophet's (peace be upon him) prohibition of buying and selling in the mosque.