Once you grab your dubious device, the seller has already got your bucks in exchange of a fake device.
You've been already and effectively cheated when those flash devices are being tested against cheats.
Whether the fraudster has somebody's dollars varies, but for that kind of a scheme they're able to just hide in plain sight. If 100 people don't test the device (and it works for months or years) and 1 person does, they have a 5-star rating and can just eat the cost of returns. Even if everyone on Hacker News started testing devices it wouldn't make a dent in fraudulent profits.
Unless you're buying your flash device on the street and paying cash, you likely can return it or initiate a chargeback.
And even if you can't undo the purchase, it's better to know whether a device is fraudulent before you start filling it up with real data that you don't want to lose.
So why state it again? This helps, for instance, people who go on vacation and takes pictures the entire trip, only to come home and realise they have only 16MB of storage, not 16GB, and their pics are gone.