There are two main ways you could sell your food or other items.
The first option is the Flea Market. You decide yourself how many copies you want to sell and to what price and/or currency.
Second option is going to your inventory, and then clicking "Sell to Vendor", which let's you handpick what items you would like to sell for a fixed prise.
you go to the flea market and on the top right hand corner there should be an option to sell, currency, all you need to do is tap it and scroll down to food select the type of food, quantity, price, and how long you want it to be on the market.