As I mentioned a couple posts ago, your tour starts off by buying a ticket. There's a million combinations - 2 day, 3 day, boat rides, helicopter rides, uptown, downtown. Generally, tourists will go for a standard all loops, 2 or 3 day pass. You can buy these online, through your hotel, through various travel agents and third parties, but the majority of people simply buy them from the guys hawking them on the street.
The first thing you need to know is that these guys are paid 100% (or very close to it) on commission. Because of this, they are the purest hustlers of anyone in the game, and have absolutely no loyalty to the company or to you, the customer. You can't really blame the ticket sellers - when the sales model of a business is "you don't sell, you don't eat" that's how it goes.
The take varies depending on the tour, but it's usually about 10 cents on the dollar of your ticket price. Of course, that 10 cents changes to a 100 cents if they can simply take your cash, put you on the bus, and never enter any records of it in the computer. If you're getting tickets for anything other than the bus ride this is tough, but if you're simply hopping on what's right in front of you, the guides usually don't care a whole heck of a lot about checking tickets. Get caught at that you're fired, but pull it off and you make a day's pay in 10 minutes. There's other, smaller, scams - charging for children's tickets instead of adults for instance - but all basically involve getting your cash and not telling the company about it.
Now, there's something else ticket sellers do all the time which is to tell you whatever you want to hear - and omit anything you don't want to hear - in order to transfer the cash from your pocket to theirs. You hear "buses all the time, boat ride, free museum, go anywhere in the city whenever you want to" but when you get down to what you can and can't do in practicality, you'll find you've paid for a lot you might not be getting. This isn't really a scam, but isn't really great customer service either.
Of course, the second thing you have to know is that nobody is at all interested in customer service. The tour bus business does not really have repeat customers, and as a result, customer service does not add anything to the bottom line - especially for the ticket sellers. As far as the ticket sellers go, you see the bus, they get your money and that's a wrap. Refund? They don't know the meaning of the word. What are you going to do, complain to the company? They don't want to give you your money back either. There's perhaps a little bit of concern for good word-of-mouth or online reviews, but as there's now only one company, that's pretty much not a worry anymore either. Once your money leaves your pocket, you have no leverage. My best advice is to always pay with a credit card, and don't be afraid to cancel the charges if you aren't happy.
I'm less familiar with the Red Bus (Grey Line), but the ticket sellers used to discount at the Blue Bus (CitySights) heavily, especially on the single ride night tour where every extra dollar is profit (it costs just as much to run a bus with 5 people on it as 50 people). It had gotten to ridiculous proportions until one time a guy who had already bought a whole bunch of tickets for his group from the hotel decided to ask about the prices with the guys on the street to see if he got a good deal. The street guys told him to return the tickets to the hotel, get their money back, come back to him and he'd sell them the tickets at a discount. The guy told the hotel, the hotel told the company, and the word came down that there was absolutely no discounting of tickets anymore. Doesn't mean you can't still bargain, but it's certainly not as-of-right anymore.
Ticket selling is a tough gig but you'd be surprised as how much someone can make. The top guys at both places made over 6 figures yearly. Now, that's an experienced person who knows the game inside and out and hustles all day, every day, 365 days a year - there's also plenty of guys who don't even make enough for lunch. But it's something I always point out as an example when people of a different kind of background complain about the lack of jobs and opportunity in New York. You want money? It's there - you don't need a fancy degree, connections, or any particular skill. Hell, you don't even need a Social Security Card or a firm command of the English language. All you got to do is throw on a yellow jacket, stand out in Times Square, and hustle like hell. So either get to it, or stop your complaining.
As with most everything in New York, the profession has its ethnic skews. A disproportionate number of the ticket sellers (and many other street vendors) are West African - just like a disproportionate number of the guides are Jewish, and the drivers Chinese, but those are other stories for other days.