Exactly. If you can sell one of your kidneys for $10,000 (or $5,000, or $20,000, doesn't matter) and you're a down-on-his-luck guy whose house is about to be foreclosed on, or who is running into the bottom of the barrel trying to support his family, you'll probably do it.
The people who don't have to do it to financially support themselves won't do it, because people simply aren't going to part with organs in order to make quick cash.
In other words, it wouldn't be about people helping other people. It would be about economic stratification getting skewed like crazy because poor people would be selling off whatever organs they might be able to sorta do without.
Heh, coming from a third world country, I've heard plenty of real stories about organ selling. Believe it or not, people are very much willing to part with organs for money. It's not because they're stupid. It's just that when you grow up illiterate, never leaving your home state, you believe the words of some guy with more money than you could possibly imagine.
Some poor people are both horribly lazy and desperate for money. Destitute people fight over money, divorce, steal, betray family members/close friends just for money. Let's put it this way: In some countries, poverty-line wages are about $0.20 an hour. Imagine how much say, $30K would offer. It means that they'd never have to work for years.
With that kind of motivation, you'd be willing to believe any lies and half-truths an organ buyer would tell you. If you told them they'd need only one lung to live, they'd probably sell it if it means they'd never have to work again. Hell, some people would sell their testicles, arms, feet, whatever they deem unnecessary if there was a demand for it.
Also put it this way - loan sharks are known to physically injure or kill people who don't pay debts in time. Selling organs is a way around borrowing from them, without debt. Legalizing organ selling means that loan sharks could just grab a debtor, cut out his kidneys, and sell it off to repay a debt.
Organ donations are a good thing. Controversial on a personal scale, but good overall. Organ selling is almost always bad.