Market makers are buying - their job is to always buy or sell stock from everybody. They (generally computers, but humans traditionally) will always buy your stocks, or sell you stocks. Their algorithm is simple: buy for $.10 (or some other tiny number) less than you sell - if the amount of stock owned is too low raise the price, if the amount is too much lower the price. They pretty much always make money in the long run.
For agreeing to do this (and having the capital to do it) the Market Maker for a stock typically secures certain benefits from the market in respect of that stock.
For a very popular stock on a typical day the market maker isn't really necessary. Your trades would absolutely execute immediately based on positions other people wanted.
When your stock is more thinly traded, or when things are a bit frantic the market maker is your saviour. When everybody and their dog is selling, the market maker will buy anyway.
Under some circumstances market makers can signal they intended to cease to make a market for specific stocks. When the market makers exit, all hobbyists should make sure they are gone too. Once there is no market maker for the stock you're holding, you will need somebody else to actually take the other side of your trades. "Prices" without a market maker are just a guess, there may be nobody actually promising to take your stock at any price, even if the last trade was for $1.40 your stock might be not sell even at 14¢. This makes for an exciting space in which to gamble with money you can afford to lose if you really know what you're doing, otherwise it's just a way to throw money away.
