If you are a real savvy daytrader, you can use arbitrage to make use of the different prices offered by market makers.
I have never done it, and I have heard its much more difficult to do it now that inefficiencies such as that are quickly corrected, so you are talking about a few seconds to act on it.
Anyways, I think you are not that far off with your concern. I have heard others talk about market makers fucking around with the stock price and I think there is some truth to it.
Think about this scenario (which is not that far off from reality!)
A big holder of a stock decides to dump all of his holdings (say 5M shares, of the total 65M common out there)
He wants to slowly release his shares to prevent the slobs like you and I from catching on and dumping our small positions (1K or so) as he unloades his.
He is going to have the market maker continue to sell shares at the market in the hopes of getting as much as possible for his shares.
If he just was like any of us slobs and put in a market order to sell 5M shares on a stock with only 65M total and average daily volume of say 300-400k, the stock would drop in HALF in a matter of minutes as the market maker certainly does not want to get stuck with that many shares.
FYI: I know someone who worked for a market maker as a runner and he told me the market makers make zillions!
And, trust me they are not making zillions on the small spread on each trade either! They control the market and the price!
Just realize what is going on and adjust to it. Realize the market is not rationale and there are people screwing around all the time with the price.
cheers.