Avoid buying flash drives on Ebay from Hong Kong, China, Korea etc. They are almost garaunteed to be fakes. They are sold as having a capacity of anything from 512MB to 32GB but actually have a lot less. Some are counterfeits of well known brands but most are unbranded.
Buyers have sometimes discovered months later that data saved to these is corrupt or not on the drive at all. Scammers (not necessarily the person who sold the device - it may be someone further up the supply chain) have used software intended for legitimate OEM use to provide memory information to the controller to supply false information about capacity to the controller (and therefore to Windows) so that the device appears to have the capacity claimed by the seller when you view the properties in Windows.
As long as you don't try to store more than the device's real capacity the drives usually work ok (at first until, you delete old files and add new ones). If you only store small files it will work up to the real capacity but everything beyond that will be lost although it appears to be being written to the drive. This sort of scam has led to many people losing data as it has been written to non-existent addresess on the device.
Some known sellers of fake flash drives have been removed from Ebay but there are more out there. Sellers currently supplying fake or suspect drives (ones that I tested and don't check out) include: onion.ginger, i.c.distribution, top789988, i.t.smart_electronic, dengkimz and chaiching28. If you want to check out flash drives there are a number of tools available to download free from the Internet. Among the tools I have found so far are:
I've found h2testw_1.4 the most useful so far.