After production, the most important part of the game is managing your supplies. You need to be able to keep a mental note of the different markets and the different ingredients that they hold. At the beginning of the game, you are probably going to have fairly low reserves due to your small budget. You'll have to deal with this by only buying essential ingredients and keeping basic reserves. Just have a five week supply for anything besides sugar, milk, or cacao. Obviously feel free to stock up on special deals and fruits in season. That's just smart planning. As you get more successful, you will find that you have more wiggle room to buy in bulk.
Remember that your goods can and will expire if you don't put them into use. Don't bother buying much of anything exotic if you don't plan to produce it you finish the order.
Always mouse over the ingredients. Your ledger keeps track of your purchases and makes a note of recent prices. This is the really quick way to find out if you are getting a good deal on something.
That said, deals aren't that important. The only really noticeable deal revolves around whether fruit is in season or not. You can find some great deals, but by the time you are dealing with such ingredients you will be past the point of counting small change. You'll still be able to make a profit as long as you don't catch it at the absolute peak price.