I started doing this a while ago.
There are a lot of fake memory cards going around on ebay. You can tell they are fake because they are going for a lot less (a third or less of the price, exact amount varies) than other places. They actually are just hacked to be less capacity than they claim. You can verify the exact capacity by using a program called h2testw.
I buy a few cards, wait for them to ship, dispute the transaction, and usually I get the money back without having to send back the item. (Once I had to send it back, but ebay paid for the shipping. Usually not, though.)
Viola, free memory cards. If you have paypal credit, it's even better, as you don't have to pay until later and you may cancel it before the payment is due. I'm ripping off scammers, so no ethical problems either.
You should not expect such cards to be reliable.
The reason this is possible is because of the way flash memory is manufactured now: the chips are produced with faults, then connected to a microprocessor that runs sophisticated proprietary algorithms to identify the faulty memory cells and reliably distribute stored files across the working memory cells, and also report to both the manufacturer and consumer the total capacity of those working cells. This way a single silicon wafer produced with imperfections can yield hundreds of sd cards ranging from mega...
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.