Never leave eBay feedback for electronics that use flash memory for data storage, until you've fully tested the device. This may seem obvious to some people, but a lot don't seem to realise the risks.
The salient information is that various manufacturers produce and sell chips that can be altered to report false details to computers they're plugged into. A device that shows itself to be, say, 8Gb may only be a few hundred megabytes in true capacity — and due to the way Windows and other operating systems buffer data storage, the first time you use it with a few small files you might not run foul of trying to write data to an area on it that doesn't exist. It may even appear to write and to be safely removed, with all appearing fine until you try to read data back from it. However, the drive isn't necessarily safe to use even if you only put a few files on, as the filesystem doesn't necessarily try to place data at the start of the space.
Fake flash drives and MP3 players are particularly big business on eBay, with overseas sellers operating with impunity from legislation designed to protect domestic consumers. That's not to say some fakes aren't sold within the UK, but most come from the countries they're manufactured in under the guise of savings to be had by buying direct.
I bought one out of curiosity, knowing it was almost certainly fake from the price. It turned out to be a 1Gb chip rather than the 2Gb it was programmed to appear as, but additionally about a quarter of the writable area was defective — the chip was probably sold cheap to scammers due to failing quality control testing past a certain point.
[Due to the way flash memory is manufactured, a stick sold as 2Gb might contain a 4Gb chip — but only the first half of the area for data tested reliable, hence an honest retailer locking it to 2Gb and only selling it as such. Similar situations exist with processors, where they're rated to run reliably at — and thus often locked to — certain speeds, and with other products. Don't assume that any flash drive (or hard drive) you own doesn't have duff areas, because modern storage tech is deliberately designed to route around flaws that occur in the manufacturing processes.]
Anyway, a low-level format of just the good sectors achieved a formatted capacity of 744Mb, which helpfully enough allows a CD worth of data (say, a film) plus a bit of software to fit. It's not useful for situations where reliability is of particular concern, of course, but neither are any consumer data storage technologies. The drive's been tested and read and recovered data fine thus far, and it's now probably as stable as any of the other flash drives I've accumulated.
Unless you have similar curiosity I wouldn't recommend this — you could get a chip with a far smaller capacity, or be unable to find software to low-level format it. The device I have reports as an iCreate 5122 but the versions of the production tool commonly in circulation for those don't even register it — whereas a current beta, labelled as IC5128, does. It's likely it isn't a 5122 but that this, along with the rest of the what's programmed onto the chip, is a fake ID.
I also wouldn't recommend complaining to the seller (you'd end up paying postage to return an item if that complaint escalated to a claim) or assuming that you're safe because a seller has good feedback — many people don't want to receive retaliatory negative feedback, or leave feedback before they realise there's anything wrong. You might have more joy with reporting an item as lost — it's unlikely the seller has sent via a trackable method, since that would eat into the profits of their fraudulent business venture — but I couldn't possibly comment on that...
You can check a drive, MP3 player or memory card legitimately has the space it says it has with a small command-line utility called testdrive.exe from schnapp.org.
Other technical resources:
(Always remember to scan software for viruses, whether it comes on a CD with new hardware or you download it from the web. Also, as a more general tip, if taking photos of an important event, take them on multiple smaller cards rather than one large one — if one memory card goes, you'll still have the rest.)