Is it permissible for a woman to increase the price of vegetables she buys from the central market for her friends without their knowledge, given that she knows she bears the cost of the spoiled vegetables while they take the good ones, and they sometimes take more than her, even though the price will remain cheap and fresh?

You are not allowed to increase the price unless your friends are aware. If you do not wish to provide the service to them for free, then inform them that you will purchase the goods for yourself and then sell them to them at a mutually agreed price. Alternatively, you can buy for all of you and request a known fee in exchange for this service.