Unfortunately that's not the way it works, this is a very efficient market. Would you pay 10$ for a steam game if it could be had for 3.99$?
take your example with the apple seller
The apple grower (seller) takes product to a market place (NYSE) and says I'm going to sell it for 5c per apple, the ask.
To create a market, the Market Maker like Knight will now put in a bid for 4 c because that is their purpose, to create liquidity in a market place.
Now we have 2 choices. the seller can decide to sell that apple to the Market maker for 4c or he can just wait around until a Buyer comes in and buys it from him at 5c.
Choice 1: the seller sells it to Knight at 4c. Knight now owns the risk on the apple potentially falling further but in the mean time, the Original seller sold his apple and has none left. Knight will now put an offer , the ask, on the apple they bought for say 4.5 cents (to help move it faster). They will also at the same time lower their Bid in case more apples are for sale down to 3c.
Choice 2: Another buyer comes in and buys the apple from teh seller at 5c. The buyer gets his apple at 5c, the seller sells at 5c. The market maker is completely not involved in this transaction. What the market maker will do though, is now set a sell price at 6c to create a market place.
Now where the market maker potentially can overstep a smaller investor is that the market maker obviously knows the best prices since they're making the market and see what's out there. if they are desperate to dump the apple they bought at 4c and try and sell it at 4.5c and another individual seller comes in at 4.4c, the algorhythm may then say "shit the market is moving down, look for the best price I can sell my position at" If a buyer steps in at 4 cents, the algo will automatically offload the position before the seller can make that decision. But that's why you have things called limit orders or market orders. Limit makes sure you get what you want if the price moves the way you want.
the exchange in all of this is simply a market place. They are not the middle men. They provide for you a service where you have a market place to trade your goods and charges you for the right to trade on the exchange. Much like if you were in a farmers market, you'd have to rent the particular stall.