There is no incentive for MM's to manipulate.
First, they get paid on the spread, not on the share price.
Second, they get paid both on the sell side and the buy side for each trade.
Third, the market is automated, no people decide how to manipulate stinky pinkies.
But mainly, it is because they get paid on the spread only.
If an MM sets their ask too high, other MM's will get the trades.
If they set the bid too low, other MM's will get the trades.
If you offer to buy shares or sell shares within an MM's spread, the trade is made. They HAVE to fill the trade by law.
If you offer to buy a penny stock for a dollar, you will get it for a penny, IF it is within an MM's spread.
Lastly, every trade leaves an audit trail. Every transaction is recorded - who sold, who bought, at what prices, when, and by which MM's. Computers are used to audit these transactions to sniff out any problems. No MM could get away with 'cheating' because the fines would be thousands of times what could be made on a pinky. If an employee of an MM decided to cheat, he/she would not be an employee for long, BECAUSE everything is recorded, forever.
If anyone thinks differently, it sucks to be you. You don't understand how the system works, but you give it your money. You don't seem to understand it is buyers and sellers that change the prices. MM's merely perform the transaction. Without them, you would seldom be able to buy OR sell stocks.
