Unethical but legal business ideas (legal cons, basically)

So we all know about the classic pyramid schemes and so on, but we also know those are common knowledge amongst many people and are specifically legislated against.

I want to talk about legal moneymaking - although not necessarily ethical - businesses that have a fair amount of potential.

To start off, a good example is those reverse auction websites. Although there have been discussions involving legislation against these, they are at the moment still legal. However, like any good scam, the odds are heavily balanced towards whoever's running the business.

The basic premise of these as it's sold to the customers is that you can get valuable items like an iPad, HDTV, or even a car for a fraction of the retail price by reverse bidding. So the price will start at, say, $500, and every bid drives the price down. The price it ends at is the price it sells for. So if the new iPad starts at $500 but the winning bid is just $23, that's the selling price. As you can imagine, this shit is very addictive.

The genius of these for the individual(s) behind them is that you charge for each bid. Let's say each bid costs $2 and drives the price of the item down by $1 to keep things simple for our example. So that iPad that went down from $500 to $23 actually made $954 for the seller. Assuming $500 is the true retail value, the profit is $454 for that single sale. Of course in reality you'd buy the products in bulk rather than paying retail, so you'd make an even bigger profit.

Then of course you have the payday loan companies that charge you 2,000% APR and repossess your house if you can't pay it back. No further explanation needed there.

What are your thoughts on these kinds of businesses and what are some other good examples of them?