There are prices for certain things like your own skills or food. But if you have to sell something like food that you can't store for long time that means you have to check all the time if the price changes. In my case i have to check the price of everything i sell everyday and everytime I sell.

Prices change from the morning to the afternoon. If the dollar price change hard you have to stop or you will gamble

As always one of the points is that you have to buy the same product when you sell it and if you don't check you won't get a profit, in the worst case you can't buy it again